Grant, DM

1997

Dynamic Nuclear Polarization of Nitrogen-15 Benzamide

Hu, J.Z.; Zhou, J.; Yang, B.; Li, L.; Qiu, J.; Ye, C.; Solum, M.S.; Wind, R.A.; Pugmire, R.J. and Grant, D.M.
SOLID STATE: Nuclear Magnetic Resonance, 8:129-37(1997). Funded by US Department of Energy/Basic Energy Services and ACERC.

A N-15 dynamic nuclear polarization (DNP) experiment is reported in which a N-15 DNP enhancement factor of approximately 2.6 x 10² is obtained on free radical doped samples of 99% N-15 labeled benazmide. The free radicals BDPA (1:1 complex of alpha, gamma-bisdiphenylene-beta-phenylallyl with benzene) and DDPH (2,2 -Di (4 - tert - octylphenyl) -1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T1 values of the samples. The combination is solids of a very low natural abundance, 0.37%, a small gyromagnetic ration, and a log spin - lattice relaxation time for N-15 nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the N-15 DNP experiment on samples containing free electrons.

Solid State N-15 and C-13 NMR Study of Several Metal 5-, 10-, 15-, 20-Tetraphenylporphyrin Complexes

Strohmeir, M.; Orendt, A.M.; Facelli, J.C.; Solum, M.S.; Pugmire, R.J.; Pery, R.W. and Grant, D.M.
Journal of the American Chemical Society, 119:7114-120(1997). Funded by National Institutes of Health, US Department of Energy.

The principal values both the C-13 and N-15 chemical shift tensors are reported in the Zn, Ni, and Mg 5, 10, 15, 20- tetraphenylporphyrin (TTP) complexes. The principal values of the 15N chemical shift tensors were obtained from static powder patterns of N-15 enriched samples. Due to overlap between the powder patterns of the different carbons, the C-13 values were obtained using the recently developed magic angle turning (MAT) 2D experiment on unenriched materials. The measured principals values are presented along with theoretical calculations of the chemical shift tensors and a discussion of the effects that the metal bonding has on the chemical shift tensors in these compounds. Both the isotropic chemical shift and the principal values of the N-15 chemical shift tensor are nearly identical for the Mg and Zn complexes. The N-15 isotropic chemical shift for the NiTPP, however, changes by nearly 80 ppm relative to the Mg and Zn values, with large changes observed in each of the tree principal values. Calculations show that the differences between the N-15 chemical shifts are almost entirely determined by the metal-nitrogen separation. In addition, both the experimental data and the calculations show only very minor differences in the 13C chemical shift tensor components as the metal is changed.

A High-Resolution 3D Separated-Local-Field Experiment by Means of Magic-Angle Turning

Hu, J.Z.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 126:120-26(1997). Funded by US Department of Energy.

A 3D separated-local-field (SLF) experiment based on the 2D PHORMAT technique is described. In the 3D experiment, the conventional 2D SLF powder pattern for each chemically inequivalent carbon is separated according to their different isotropic chemical shifts. The dipolar coupling constant of a C-H pair, hence the bond distance, and the relative orientation of the chemical-shift tensor tot eh C-H vector can all be determined for the protonated carbons with a single measurement. As the sample turns at only about 30 Hz in a MAT experiment, the SLF patterns obtained approach those of a stationary sample, and an accuracy in the measurement similar to that obtained on a stationary sample is expected. The technique is demonstrated on 2,6-dimethoxynaphthalene, where the C-13-H-1 separated-local-field powder patterns for the six chemically inequivalent carbons are clearly identified and measured. The observed dipolar coupling for the mothoxy carbons is effectively reduced by the fast rotation of the group about its C3 symmetry axis. The average angle between the C-H bond direction and the C3 rotation axis in the OCH3 group is found to be about 66°.

N-15 CPMAS NMR of the Argonne Premium Coals

Solum, M.S.; Pugmire, R.J; Grant, D.M.; Kelemen, S.R.; Gorbaty, M.L. and Wind, R.A.
Energy & Fuels, 11:491-94(1997). Funded by ACERC and US Department of Energy.

N-15 NMR data are reported for the Argonne Premium Coals. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Delectability of different types of nitrogens is discussed in terms of cross-polarization dynamics together with effects of the large chemical shift anisotropy that is found in different types of nitrogen functional groups. Data on acid-treated coals confirm the presence of pyridinic type nitrogens that were not observed in a previous N-15 NMR study of coals.

N-15 NMR Spectroscopy of Coals and Pyrolysis Products

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Fletcher, T.H. and Wind, R.A.
Proceedings of the 9th International Conference on Coal Science, Essen, Germany, September 7-12, 1997. Funded by ACERC, US Department of Energy/University Coal Research and New Energy and Industrial Technology Development Organization.

N-15 NMR spectra are reported for a number of coals from Pacific Rim countries. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Detection of different types of nitrogen species is discussed in terms of cross polarization dynamics together with the effects of the large chemical shift anisotropy that are found in different types of nitrogen functional groups. Significant differences are observed in the types of nitrogen present in these coals and these variations are associated with coal rank as has been in a previous study of the Argonne Premium coals. We have also begun to examine pyrolysis char and tar samples. The NMR data indicate that significant differences exist between the types of nitrogen structures observed in coal, char and tar samples. These differences suggest that different mechanisms may exist for nitrogen release from tar and char samples.

1996

Effects of Hydrogen Bonding in the Calculation of N-15 Chemical Shift Tensors: Benzamide

Facelli, J.C.; Pugmire, R.J. and Grant, D.M.
Journal of the American Chemical Society, 118(23):5488-5489, 1996. Funded by National Institutes of Health and US Department of Energy/Basic Energy Services.

In a recent paper the principal values of the N-15 chemical shift tensor in benzamide have been reported. The tensor principal values have been measured using the dynamic nuclear polarization (DNP) experiment and also by cross polarization (CP) in static sample of N-15-benzamide at room temperature. Both experimental approaches produce very similar principal values. The values obtained by fitting the CP static spectrum with the Powder method are the following: delta11= -176 ppm, d22= -321 ppm, and d22= -326 ppm. These values are referenced to an external sample of nitromethane and yield and isotropic shift value of -274 ppm, which is close to the -271.5 ppm MAS (Magic Angle Spinning) value and the liquid values of -278.4 ppm in DMSO and -282 ppm in CDC13, from the literature. Because the measurements were done in a disordered sample no information was available on the orientation of the principal components and only reliable quantum chemical calculations of the tensor would allow these assignments to be made.

The results presented in this communication highlight the importance of including HB and, in general, intermolecular interactions in the calculation on N-15 chemical shifts tensors. This has been observed before in a previous study of the N-15 chemical shifts tensors of uracil but it appears that the magnitude of the effect is greater for an amide nitrogen than for the aromatic nitrogens in uracil. Further evidence of the importance of including intermolecular interactions in the calculation of N-15 chemical shifts tensors can be found in a recent study of these tensors in a series of heterocycles. For these compounds the RMS between experimental and calculated values, which do not include intermolecular effects, is approximately 30-40 ppm. The large RMS value is comparable with the values obtained here when the HB is neglected and almost one order of magnitude larger than the RMS obtained when the HB is included in the calculations. This may indicate that the findings presented here are not limited to benzamide, but are a general feature in the calculation of N-15 chemical shift tensors. The extreme sensitivity observed in the tensor components from HB indicates that the study of N-15 tensor components may be used to complement the determination of the tertiary structures of molecules of biological significance in solid state samples and/or in frozen solutions. It is well known that the HB is the principal governing factor in the tertiary components to these interactions makes N-15 tensors premier indicators of biomolecular structure and portends high value for these NMR methods. Finally, due to the high sensitivity and specificity of tensors, this work emphasizes some of the advantages of measuring tensor components instead of the traditional isotropic.

Carbon-13 Chemical Shift Tensors and Molecular Conformation of Anisole

Facelli, J.C.; Orendt, A.M.; Jiang, Y.J.; Pugmire, R.J. and Grant, D.M.
The Journal of Physical Chemistry, 100(20):8268-8272, 1996. Funded by ACERC and US Department of Energy/Basic Energy Services.

The first direct measurement of the ortho steric effect of the methoxy group on the C-13 chemical shifts in anisole is reported. The ortho steric effect on the isotropic C-13 chemical shifts was obtained from a low-temperature MAS spectrum, and the effect on both the isotropic and the tensor principal components was determined from a low-temperature 2D magic angle turning (MAT) experiment. Form the low-temperature MAS spectrum, the C-13 chemical shift of the ortho carbon cis to the methoxy carbon is found to be 7.0 ppm from the low-temperature MAT experiment, a 6.8 ppm decrease in the chemical shift is observed in the isotropic chemical shift, while the effects on the difference (cis minus trans) between the individual tensor components are measured to be -9 ppm in delta1, 1 ppm delta22 and -14 ppm in delta33, in reasonable agreement with the results of a previous linear regression substituent analysis on several di- and trimethoxybenzenes. Comparison of the experimental results with calculations, including thermal averaging considerations, further demonstrates that at room temperature the methoxy group in anisole undergoes stochastic jumps between the two equivalent planar configurations. This work demonstrates the feasibility of using the low-temperature MAT experiments at low temperature to measure the principal values of the C-13 chemical shift tensors in molecules that are liquids at room temperature.

Glass and Crystal Formation in Binary Aromatic Mixtures: A Mechanism for Reducing Spin-Lattice Relaxation Times

Wang, W.; Pugmire, R.J. and Grant, D.M.
J.Phys. Chem (in press), 1996. Funded by US Department of Energy/Pittsburgh Energy Technology Center and US Department of Energy/Basic Energy Services.

The phase diagram constructed form differential scanning calorimetry data indicated that the binary mixture of dibenzofuran (DBF) and hexamethylbezene (HMB) forms a simple eutectic system. Comparative studies of proton T1 Values for the annealed and quenched samples show the annealed material can be best described as a two-phase mixed crystals, while a rapidly quenched sample is a combination of a metastable one-phase glass and two-phased mixed crystals. It is found that glass formation is the key to T1 reduction of DBF in the HMB doping technique reported previously. The interesting rends in the TA and the relative spin population of DBG is explained with the competition between glass formation and crystalline phase separation.

1994

Magic Angle Turning Experiments for Measuring Chemical-Shift-Tensor Principal Values in Powdered Solids

Hu, J.Z.; Wang, W.; Liu, F.; Solum, M.S.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
J. Magnetic Resonance, 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.

The magic-angle-turning (MAT) technique introduced by Gan employs slow (ca. 30 Hz) rotation of a powdered sample at the magic angle, in concert with pulses synchronized at 1/3 of the rotor period, to obtain isotropic-shift information in one dimension of a 2D spectrum. The other dimension displays a slow-spinning-sideband powder pattern that, at the low rotor frequencies employed, resembles the stationary-sample powder pattern. The MAT method is very effective for measuring chemical-shift principal values in compounds where spectral overlap precludes the use of 1D methods. Previous MAT implementations are reviewed, and it is shown how a new phase correct MAT (PHORMAT) pulse sequence overcomes many of their limitations. This new pulse sequence produces a spinning-sideband-free isotropic-shift spectrum directly as a projection onto the evolution axis with no spectral shearing. Only two purging operations are employed, resulting in a higher signal-to-noise ratio. Pure absorption-absorption phased 2D spectra are produced. Flat 2D base planes result from an echo sequence which delays acquisition until after probe ring-down and receiver recovery. The technique used for synchronizing the pulses to 1/3 the rotor period without relying on absolute rotor-frequency stability is described. The PHORMAT spectrum of methyl a-D-glucopyranoside is presented. The data are analyzed with an emphasis on the quantitative accuracy of the experiment in measuring chemical shift tensor principal values and determining the relative number of spins of each type present. The FID data from the spectrometer acquisition are fitted with numerical simulations that employ a banded-matrix method for calculation spinning sideband amplitudes. The chemical shift principal values, measured in methyl a-D-glucopyranoside with the PHORMAT method, are compared with those from a single-crystal determination of the full chemical shift tensors. The two measurements differ by an rms average distance of only 0.57 ppm.

The N-15 Chemical Shift Tensors of Uracil Determined from N-15 Powder Pattern and N-15/C-13 Dipolar NMR Spectroscopy

Anderson-Altmann, K.L.; Phung, C.G.; Mavromoustakos, S.; Zheng, Z.; Facelli, J.C.; Poulter, C.D. and Grant, D.M.
Journal of Physical Chemistry, 1994 (in press). Funded by National Science Foundation.

The N-15 chemical shift tensors of uracil are detemined by N-15 powder pattern techniques. The principal values of the N-15 uracil tensors are obtained from the spectra of [1-N-15]uracil and [3N-15]uracil, and the tensor orientations are determined from the spectrum of [1,3-N-152, 2-C-13]uracil with the inclusion of dipolar interactions. Ambiguities in the orientational assignments are resolved using molecular symmetry considerations and results of ab initio calculations of the N-15 chemical shielding tensors. The N1 nitrogen has principal values of 196 ppm, 114 ppm, and 30 ppm and the N3 nitrogen 200 ppm, 131 ppm, and 79 ppm with respect to N-15H4NO3. The components with the largest chemical shifts lie approximately along the N-H bonds. Including the effect of intermolecular hydrogen bonds on the theoretical calculations improves in a significant way the agreement between the calculated and experimental chemical.

Solid State C-13 NMR, X-Ray and Quantum Mechanical Studies of the Carbon Chemical Shift Tensors of P-Tolyl Ether

Facelli, J.C.; Hu, J.Z.; Orendt, A.M.; Arif, A.M.; Pugmire, R.J. and Grant, D.M.
Journal of Physical Chemistry, 1994 (in press). Funded by US Department of Energy and ACERC.

This paper presents a detailed study of the principal components of the C-13 chemical shift tensors in p-tolyl ether. The tensor components of a relative large number of carbon atoms are measured by using the two-dimensional magic angle turning (MAT) technique that allows for the determination of the principal components of the chemical shift tensors in powders. Theoretical calculations of the C-13 chemical shieldings, using the X-ray molecular geometry, are used to assign the NMR resonances to individual carbon nuclei. The principal values of the chemical shift tensors permit assignments that would be unreliable if only the isotropic shift information is used. The chemical shift tensors of the carbons directly attached to the oxygen atom are very sensitive to the structural and electronic properties of the ether linkage. The combination of the C-13 MAT experiment and theoretical chemical shieldings proves to be important in the study of electronic properties and molecular structure.

Measurement of C-13 Chemical Shift Tensor Principal Values with a Magical Angle Turning Experiment

Hu, J.Z.; Orendt, A.M.; Alderman, D.W.; Pugmire, R.J.; Ye, C. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 5:181, 1994. Funded by US Department of Energy and ACERC.

The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of C-13 chemical shift tensors in powdered solids. A large-volume MAT prove with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively, can enhance measurements of the principal values of chemical shift tensors in complex compounds. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the C-13 principal values in 2,3-dimethlnaphthalene and Pocohontas coal are reported at typical examples.

The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment

Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
ACS Preprints, Division of Fuel Chemistry, 39:8-112, 1994. Funded by Pittsburg Energy Technology Center, US Department of Energy and ACERC.

The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high-resolution spectra in complex solids such as coal. MAS narrows the chemical shift anisotropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values. In and effort to address this problem of spectral overlap, many 2D techniques have been developed to simultaneously obtain the dispersion by isotropic shift, such as produced by MAS, in one dimension and the tensorial information as a separate powder patters in the second dimension. A very successful technique is the slow spinning modification of the magic angle hopping experiment recently proposed by Gan, which we call the Magic Angle Turning (MAT) experiment. This experiment has a number of advantages over earlier 2D methods. The use of very slow spinning speeds (<50 Hz) favors the quantitative polarization of all carbons and allows the use of a large volume sample rotor resulting in a typical 2D spectrum acquisition requiring less than 24 hours. The mechanical device for slow spinning is very stable and high resolution in the isotropic chemical shift dimension can be easily obtained. The MAT experiment could be done on a suitably stable MAS probe. The only disadvantages of the original MAT experiment is that data acquisition starts right after the last pulse, causing distortion in the evolution dimension (the second dimension) even if a delay as short as 20 ms is used.

In this paper, a triple-echo MAT sequence, previously described, is employed which improves the 2D baseline. Two additional experiments, using short contact times and dipolar dephasing techniques, are also employed to further separate the powder patterns of protonated and nonprotonated carbons in complex compounds. Experimental results on representative model compounds as well as coals are presented in this paper.

1993

Improvements to the Magic Angle Hopping Experiment

Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 2:235-243, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.

Several improvements to the magic angle hopping experiment first introduced by Bax et al. (J. Magn. Reason., 52 (1983) 147 are presented. A dc servo motor driven sample hopping mechanism which requires less than 60 ms to accomplish a 120º sample rotation is described. Modifications to the data acquisition process, including starting the acquisition period immediately after the second hop and acquiring a hypercomplex data set, are also presented. Principal values of the C-13 chemical shielding tensor are measured for 1,2,3-trimethoxybenzene and 2,6-dimethoxynaphthalene.

Chemical Structural Features of Pyridine Extracts and Residues of the Argonne Premium Coals Using Solid State C-13 NMR Spectroscopy

Fletcher, T.H.; Bia, S.; Pugmire, R.J.; Solum, M.S.; Woods, S. and Grant, D.M.
Energy & Fuels, 7 (6):734-742, 1993. (Presented at the Spring Meeting of the Western States Section of the Combustion Institute, Salt Lake City, UT, March 1993.) Funded by ACERC.

Soxhlet extractions were performed on the eight Argonne Premium coals using pyridine purged with argon and followed by a novel washing procedure to remove the pyridine. Mass closure (extracts plus residues) on duplicate experiments accounted for 94-96% of the original coal, repeatable to within 2%. Chemical structural features determined from C-13 NMR analyses of the extracts and residues showed more attachments per aromatic cluster for the residues, indicating a greater degree of covalent bonding in the residue than in the extract. H-1 NMR analysis of the extracts showed a gradual increase in the hydrogen aromaticity with rank, along with a maximum in the percentage of a-hydrogen in the high-volatile bituminous coals. Composite chemical features constructed from weighted averages of the features of the residues and extracts agree with many of the features of the parent coal. Chemical structural features of the extracts determined from H-1 NMR analyses agree with similar data reported previously for early coal tars during devolatilization at rapid heating rates.

Measurement of C-13 Chemical-Shift Anisotropy in Coal

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Carbonaceous Solids, Advances in Chemistry Series #229, 22:419-439, 1993. Funded by US Department of Energy, Basic Energy Services and ACERC.

The methods of available in NMR spectroscopy to obtain the principal values of the chemical-shift tensor are discussed. Applications to coal and to compounds with model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical-shift powder patterns is compared to results obtained by cross-polarization with magic-angle spinning and dipolar dephasing.

An Isotropic Chemical Shift-Chemical Shift Anisotropy Magic-Angle Slow-Spinning 2-D NMR Experiment

Hu, J.Z.; Alderman, D.W.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 31: 472, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.

High-speed magic-angle spinning has become a ubiquitous method for obtaining high-resolution spectra in polycrystalline and amorphous solids. MAS reduces a chemical-shift-anisotropy powder pattern to a single line at the isotropic shift when the sample spinning speed is larger than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the three principal values of the tensor are even more valuable. These principal values are available in the powder pattern obtained from a stationary or slowly spinning sample provided the molecule has few enough unique nuclei that the spectrum can be interpreted. Unfortunately, overlap of several broad powder patterns often prevents the separation necessary for their individual identification and measurement.

Recognizing that an equivalent of the hopping experiment could be achieved without stopping the sample, Gan has recently demonstrated an elegant technique that employs pulses spaced at one-third of the rotor period to produce 2D spectra with an isotropic shift projection along the evolution dimension and an undistorted slow-spinning-sideband powder pattern in the acquisition dimension. The pulse sequence produces an isotropic shift dimension completely free of sidebands even in the slow-spinning regime. Because it requires only a slow continuous rotation of the sample, Gan's experiment is much easier to perform than the hopping experiment. However, in the process of projecting the magnetization onto the longitudinal axis twice during the rotor period, only one-fourth of the magnetization is retained and the sensitivity of the experiment is reduced accordingly. Described here is a technique which uses the same slow rotation of the sample about an axis at the magic angle, but instead applies five pi (180º) pulses to the magnetization Precessing in the transverse plane. The pulse sequence produces a result similar to that of Gan's experiment without the sacrifice of any magnetization to projections out of the transverse plane. This "5p" pulse method can be viewed simply as a constant-time version of Gan's experiment, with resultant advantages and disadvantages to be discussed.

Comparing, Modeling, and Assigning Chemical-Shift Tensors in the Cartestian, Irreducible Spherical, and Icosahedral Representations

Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Journal of Magnetic Resonance, 101:188-197, 1993. Funded by US Department of Energy, Division of Chemical Sciences and Office of Basic Energy Sciences.

A measure of the difference between two chemical-shift tensors is developed by defining the scalar distance between them. Chemical-shift tensors are treated as functions whose domain is the surface of a sphere and the mathematical definition of the quadratic distance between two functions is invoked. Expressions for the distance between two chemical-shift tensors are developed in the Cartesian and irreducible spherical representations and in a new icosahedral representation. A representation wherein the chemical-shift tensor is specified by the shifts when the magnetic field is along six directions defined by the vertices of an icosahedron is developed and its properties are discussed. The expression for the distance between two tensors is found to be particularly attractive and useful in this icosahedral representation. The distance between tensors computed in the icosahedral representation is useful in fitting linear models to tensor data. It is shown how such fitting can contribute to the assignment of tensors obtained from single-crystal studies. A quantitative figure of merit useful for comparing multiple assignment possibilities is developed. The results derived are applicable to any physical phenomenon described by real zero-rank and second-rank tensors.

Chemical-Shift-Chemical-Shift Correlation Spectroscopy in Powdered Solids

Hughes, C.D.; Sherwood, M.H.; Alderman, D.W. and Grant, D.M.
Journal of Magnetic Resonance, 102:58-72, 1993. Funded by US Department of Energy.

Sample reorientation during the mixing time of a two-dimensional NMR exchange experiment allows the measurement of chemical-shift tensors in powdered samples whose one-dimensional powder patterns are too complex to reliably analyze. This technique has the advantages that the chemical-shift anisotropy has an unscaled representation, the artifacts caused by cross polarization, spin relaxation, and partial sample orientation can be accounted for, the experiment is relatively easy to perform, and no spinning sidebands interfere. The method is useful for sorting out the principal values of overlapping chemical-shift tensors.

Determination of Molecular Symmetry in Crystalline Naphthalene Using Solid State NMR

Facelli, J.C. and Grant, D.M.
Nature, 365:325-327, 1993. Funded by US Department of Energy, National Institutes of Health, Utah Supercomputing Institute and the State of Utah.

Diffraction techniques have shown that the crystal structure of naphthalene has a unit cell with Ci symmetry. These studies were unable, however to resolve any departure of the molecular structure from the D2h symmetry observed in the gaseous state. We found recently that the solid-state C-13-nuclear magnetic resonance (NMR) chemical shifts for naphthalene exhibit the Ci symmetry of the unit cell. If these chemical-shift data reflect structural distortions of the molecule, rather than simply intermolecular effects on the shifts owing to the Ci symmetry of the environment of each molecule, one could assert that the NMR data are able to reveal structural information beyond the limits of the diffraction methods. Here we show that this is the case by performing ab initio quantum-mechanical calculations of the C-13 chemical shifts for naphthalene, and their derivatives, with respect to structural parameters. We find that intermolecular shift terms (which of necessity exhibit Ci symmetry) can account for only 30% of the maximum deviations from D2h symmetry; the remainder must therefore result from structural distortions of the molecules below D2h symmetry. This sensitivity of NMR chemical shifts to very small changes in molecular structure opens up the possibility of using solid-state NMR along with quantum-chemical methods to refine structural parameters obtained from diffraction methods.

Chemical Structural Features of Coal Chars, Tars, and Char Extracts During Rapid Pyrolysis Using C-13 and H-1 NMR Spectroscopy

Fletcher, T.H.; Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Bai, S.; Ma, J. and Woods, S.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by ACERC.

Structural characteristics have been determined for parent coals and for chars collected at different stages of pyrolysis. Recent work has focused on trying to understand the relationship between chemical structural features of the unreacted coal and the devolatilization and char oxidation phenomena. Models of coal devolatilization have recently related devolatilization behavior to the structure of the parent coal and the initial amount of pyridine extracts. Fong et al. used pyridine extraction methods to quantify the amount of metaplast formed during pyrolysis of a Pittsburgh #8 coal. These experiments demonstrated that under moderate heating conditions (~500 K/s to 873 K), as much as 80% of the initial coal was transformed into a combination of extractable material and volatiles. The work presented here is an examination of the pyridine extraction procedure of the Argonne Premium coal samples and the detailed study of the carbon skeletal structure of the extracts and the extraction residues from these coals. This is the first stage of an experimental program to examine the yield and chemical features of extracts of coal chars collected as a function of time during pyrolysis.

The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment

Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by US Department of Energy and Pittsburgh Energy Technology Center.

The chemical shift of a C-13 spin in a solid sample varies with the change of the relative orientation of the nuclei (or molecule) to the external magnetic field. This orientational dependence produces the well-known chemical shift anisotropy (CSA). The tensor values of the CSA provides a wealth of information about subtle differences in the electronic environment of the nuclei, such as the type of bond, the effects of electron devolatilization and the bond conformation as well as the dynamics of the nuclei. The principal values of the CSA can be obtained in a straightforward way when the molecule has only a few, e.g., 2-3 unique nuclei. In most substances, unfortunately, the overlap of the several broad powder patterns prevents the spectral separation necessary for individual carbon resolution and identification.

The Use of Solid State C-13 NMR Spectroscopy to Study Pyridine Extracted and Extraction Residues in the Argonne Premium Coals

Pugmire, R.J.; Solum, M.S.; Bai, S.; Fletcher, T.H.; Woods, S. and Grant, D.M.
Proceedings of the 205th ACS National Meeting, 38, no. 2, Denver, CO, March 1993. Funded by ACERC.

The relationship between coal structure and combustion behavior is a matter of on-going research in our laboratories. A great deal of effort has gone into obtaining data that is used for modeling studies of devolatilization behavior. We have also carefully studied the process of char formation. Our past work has focused on trying to understand the relationship between coal/char/tar formation as they relate to the devolatilization and char oxidation phenomena. The formation of metaplast during pyrolysis was studied by Fong and Howard in terms of extractable material obtained at different stages of the devolatilization process. We have recently turned our attention to metaplast formation in devolatilization and plan to conduct a series of experiments that will help define the formation and chemical structure of metaplast in coals of different rank.

The Magic Angle Turning Experiment. A New NMR Tool for Studying Coal Structure

Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
10th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 1993. Funded by US Department of Energy and ACERC.

The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high resolution spectra in complex solids such as coal. MAS narrows the chemical shift anistropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anistropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values.

Coal and Char Structural Parameters Derived from Solid State C-13 NMR Studies

Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
The 5th Australian Coal Science Conference, Melbourne, Australia, March 1993. (Also presented at the 5th Australian Coal Science Conference, Melbourne, Australia, November, 1992.) Funded by ACERC.

In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.

1992

C-13 NMR Techniques for Structural Studies of Coals and Coal Chars

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Chapter 10, Advances in Coal Spectroscopy, (H.L.C. Meuzelaar, ed.), Plenum Publishing Corp., New York, 1992. Funded by Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction, US Department of Energy and ACERC.

Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.

Chemical Structure of Char in the Transition from Devolatilization to Combustion

Fletcher, T.H.; Solum, M.S.; Grant, D.M. and Pugmire, R.J.
Energy & Fuels, 6(5):643-650, 1992. [Also included in the ACS Division of Fuel Chemistry Preprints, 37(2):677-684, 1992 (203rd ACS National Meeting, San Francisco, CA, April 1992)]. Funded by Sandia National Laboratories, Brigham Young University and ACERC.

Coal devolatilization experiments are generally conducted separately from char oxidation experiments, and the relationship between the chars generated in the two types of research is often ignored. However, char is one of the most important products of coal devolatilization and must be characterized as a function of temperature and heating rate in a manner similar to that for gaseous behavior. In this work, the chemical structures of chars from five coals of different rank are examined, and implications on char reactivity are discussed. Chars were obtained as a function of residence time in a devolatilization experiment (1250 K, 2 x 104 J/s in nitrogen) and just subsequent to devolatilization in a laminar flame-fired experiment (1500 K, 5 x 104 K/s). Quantitative measurements of chemical structure were performed on the coals and chars using C-13 nuclear magnetic resonance (NMR). Results show that the chemical structures of fully devolatilized chars are very similar, even though a wide diversity is seen in the parent coal structures. For example, the average cluster molecular weights of the chars span a range of only 50 amu, and the side-chain molecular weights of the chars span a range of only 6 amu. The similarity in chemical structure of fully devolatilized coal chars suggests that differences in measured heterogeneous char reactivities are caused by differences in the physical structure of the char.

Selective Saturation and Inversion of Multiple Resonances in High-Resolution Solid-State C-13 Experiments Using Slow Spinning CP/MAS and Tailored Dante Pulse Sequences

Hu, J.Z.; Pugmire, R.J.; Orendt, A.M.; Grant, D.M. and Ye, C.
Solid State Nuclear Magnetic Resonance, 1:185-195, 1992. [Also included in ACS Division of Fuel Chemistry Preprints, 37(2):646-659, 1992 (203rd ACS National Meeting, San Francisco, CA, April 1992)]. Funded by US Department of Energy and Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction.

Taking advantage of the long C-13 T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in C-13 CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90% DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of his method, including the simplification of the measurement of the principal values of the C-13 chemical shift tensor under slow MAD conditions are described. The determination of the aromaticity of coal using a relatively slow MAS spinning rate is also described.

Carbon-13 Chemical Shift Tensors in Aromatic Compounds. 3. Phenanthrene and Triphenylene

Soderquist, A.; Hughes, C.D.; Horton, W.J.; Facelli, J.C. and Grant, D.M.
Journal of American Chemistry Society, 114(8):2826-2832, 1992. Funded by National Institutes of Health and US Department of Energy.

Measurements of the principal values of the C-13 chemical shift tensor are presented for the three carbons in triphenylene and for three different alpha-carbons in phenanthrene. The measurements in triphenylene were made in natural abundance samples at room temperature, while the phenanthrene tensors were obtained from selectively labeled compounds (99% C-13) at low temperatures (~25 K). The principal values of the shift tensors were oriented in the molecular fram using ab initio LORG calculations. The steric compression at C4 in phenanthrene and in corresponding positions in triphenylene is manifested in a sizable upfield shift in the alpha33 component relative to the corresponding alpha33 values at C1 and C9 in phenanthrene. The upfield shift in alpha33 is mainly responsible for the well-known upfield shift of the isotropic chemical shifts of such sterically perturbed carbons. In phenanthrene C9 exhibits a unique a22 value reflecting the greater localization of pi-electrons in the C9-C10 bond. This localization of the pi-electrons at the C9-C10 bond in the central ring of phenanthrene also corresponds with the most likely ordering of electrons described by the various Kekulé structures in phenanthrene. The analysis of the C-13 chemical shieldings of the bridgehead carbons in triphenylene provides significant experimental information on bonding between rings in polycyclic aromatic compounds. The results confirm that the electronic structure of triphenylene is best described by three fairly isolated benzene rings linked by C-C bonds of essentially single bond character. Similarly in phenanthrene, the bonding structure that correlates the shielding information may be characterized by the dominance of two benzene rings comprising the biphenyl moiety. A strong C9-C10 pi-bond with only limited pi-electron character n the C8a-C9 and C10-C10a bonds is indicated by both the experimental and theoretical results.

The Revised Structure of Bistramide A (Bistratene A): Application of a New Program for the Automated Analysis of 2D INADEQUATE Spectra

Foster, M.P.; Mayne, C.L.; Dunkel, R.; Pugmire, R.J.; Grant, D.M.; Kornprobst, J.-M.; Verbist, J.-F.; Biard, J.-F. and Ireland, C.M.
Journal of the American Chemistry Society, 114(3):1110-1111, 1992. Funded by National Institutes of Health and US Department of Energy.

The two-dimensional structure of bistramide A (a.k.a. bistratene A) has been revised. The bis-amide polyether was isolated from a Lissoclinum sp. ascidian collected in the Fiji islands. The structure was established by a combination of modern NMR methods including 2D INADEQUATE, PS-DQF-COSY, HMQC and HMBC experiments. Critical to establishing the carbon-carbon backbone was CCBond, a new program for automated analysis of 2D INADEQUATE spectra. Bistramide A demonstrated in vitro cytotoxicity towards human colon tumor HCT-116 and murine leukemia L1210 cell lines with an IC50 of 0.1 µg/mL.

Carbon-13 Chemical Shift Tensors in Aromatic Carbons. 4. Substituted Naphthalenes

Orendt, A.M.; Sethi, N.K.; Facelli, J.C.; Horton, W.J.; Pugmire, R.J. and Grant, D.M.
Journal of the American Chemistry Society, 114(8):2832-2836, 1992. Funded by US Department of Energy and ACERC.

The principal values of the C-13 chemical shift tensor were measured for the aromatic carbons of 1,4,5,8-tetramethylnaphthalene, 1,2,3,6,7,8-hexadropyrene, and pyracene using the variable angle spinning technique. Ab initio calculations of the complete shielding tensors are also reported and provided the orientation of the principal values in the molecular frame. Good agreement between theory and experiment is obtained. The chemical shift principal values and orientations of these substituted naphthalenes correspond to those found in naphthalene; the observed differences can be rationalized in terms of the strain introduced by the alkyl substituent. In the naphthalenic derivatives studied, the bridgehead carbons exhibit relatively extensive pi-electron delocalization not found in some bridgeheads in more highly condensed aromatic systems.

An Efficient Large Sample Volume System for Solid State NMR

Pugmire, R.J.; Jiang, Y.J.; Solum, M.S. and Grant, D.M.
Bulletin of Magnetic Resonance, 14:144, 1992. (Presented at ISMAR 92, Vancouver, B.C., July 1992.) Funded by ACERC.

An efficient large sample volume system has been developed to carry out MAS solid-state NMR experiments. The system components are primarily zirconia and macor and no background C-13 is observed. The stator design employs separate air bearing and drive systems and is run using dry air at a bearing pressure of about 32 psi. the rotor can be spun in a stable manner from less than one hundred Hz (with a driving pressure of 5 psi) to 4.3 KHz (24 psi driving pressure). This low gas pressure feature makes the system easy to operate. The volume of the rotor is 1.8 cm³ and it can hold 1.1 g of HMB. The S/N ratio obtained is a factor of 4.6 better than the rotor previously designed and used in our laboratory (volume 0.6 cm³ : 0.28g HMB). This increased sample size allows us to obtain the same S/N ratio in a MAS spectrum with a factor of 21 saving in spectrometer time. The time saving achieved with this rotor system is extremely useful in obtaining data on biological samples and polymers, and is especially useful when experiments on fossil fuels require the use of the Bloch delay technique. Examples of relevant applications will be discussed.

Improvements in the Computerized Analysis of 2D INADEQUATE Spectra

Dunkel, R.; Mayne, C.L.; Pugmire, R.J. and Grant, D.M.
Analytical Chemistry, 64 (24):3133-3149, 1992. Funded by US Department of Energy, Pittsburgh Energy Technology Center and ACERC.

The carbon skeleton of a molecule can be determined by using the powerful 2D INADEQUATE experiment, but the method suffers from very poor sensitivity at natural carbon-13 abundance. A computer program, described previously, has been significantly improved in its ability to recognize AB spectral patterns corresponding to carbon-carbon bonds which makes it possible to evaluate reliably spectra with rms S/N ratio as low as 2.5, i.e., nearly 1 order of magnitude below the level required for routine manual interpretation. Application of the INADEQUATE experiment to samples containing as little as 20 µmol of a compound of interest is now possible. The method is described in detail and critically evaluated by means of examples and simulations.

Applications of the Improved Computerized Analysis of 2D INADEQUATE Spectra

Dunkel, R.; Mayne, C.L.; Foster, M.P.; Ireland, C.M.; Li, D.; Owen, N.L.; Pugmire, R.J. and Grant, D.M.
Analytical Chemistry, 64:3150-3160, 1992. Funded by US Department of Energy, Pittsburgh Energy Technology Center and ACERC.

This paper illustrates the use of the program CCBond to determine the carbon skeletons of bioorganic molecules in low concentration samples. Discussed is the structure elucidation of bistramide A, a compound extracted from a Fijian Lissocllnum sp. and cholesterol in 71- and 20-µmol samples, respectively. The detection limit of the automated bond extraction is shown to be improved compared to the manual interpretation of 2D INADEQUATE spectra.

Coal and Char Structural Parameters Derived from Solid-State C-13 NMR Studies

Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
Fifth Australian Coal Science Conference, Melbourne, Australia, November 1992. Funded by ACERC.

In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.

The Use of C-13 NMR Structural Information for Predicting Coal Devolatilization Behavior

Pugmire, R.J.; Solum, M.S.; Grant, D.M. and Fletcher, T.H.
Second International Conference on Coal Structure, Krakow, Poland, September 1992. Funded by ACERC.

Recent coal devolatilization models have attempted to incorporate coal structural information to predict product yields and composition. The chemical structure of the parent coal directly affects devolatilization behavior. The parent coal structure also has implications for pyrolysis char structure and reactivity. C-13 NMR data have been used to obtain coal-dependent chemical structural data that is used as input parameters to the chemical percolation devolatilization (CPD) model. The CPD model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Data will be presented on 20 different coals and the value of NMR data in predicting gas and tar yields will be discussed. The use of C-13 NMR data will also be used to describe the relationship between chemical structure features of the parent coals and the resultant pyrolysis chars.

Chemical Shift-Chemical Shift Correlation Spectroscopy in Powdered Solids

Hughes, C.D.; Sherwood, M.H.; Alderman, D.W. and Grant, D.M.
Fifth Coal Chemistry Conference and Workshop, Rockford, IL, June 1992. Funded by US Department of Energy.

Sample reorientation during the mixing time of a two-dimensional NMR exchange experiment allows the measurements of chemical shift tensors in powdered samples whose one-dimensional powder patterns are too complex to reliably analyze. This technique has the advantages that the chemical shift anisotropy has an unscaled representation, the artifacts caused by cross polarization, spin relaxation and partial sample orientation can be accounted for, the experiment is relatively easy to perform, and no spinning sidebands interfere. The method is useful for sorting out the principal values of overlapping chemical shift tensors.

1991

C-13 NMR Techniques for Structural Studies of Coals and Coal Chars

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Advances in Coal Spectroscopy, (H.L.C. Meuzelaar, ed.), Plenum Publishing Corp., New York, 1991 (in press). Funded by Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction, US Department of Energy and ACERC.

Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.

The Revised Structure of Bistramide A (Bistratene A): Application of a New Program for the Automated Analysis of 2D INADEQUATE Spectra

Foster, M.P.; Mayne, C.L.; Dunkel, R.; Pugmire, R.J.; Grant, D.M.; Kornprobst, J.-M.; Verbist, J.-F.; Biard, J.-F. and Ireland, C.M.
Journal of the American Chemistry Society, 1991 (in press). Funded by National Institutes of Health and US Department of Energy.

The two-dimensional structure of bistramide A (a.k.a. bistratene A) has been revised. The bis-amide polyether was isolated from a Lissoclinum sp. ascidian collected in the Fiji islands. The structure was established by a combination of modern NMR methods including 2D INADEQUATE, PS-DQF-COSY, HMQC and HMBC experiments. Critical to establishing the carbon-carbon backbone was CCBond, a new program for automated analysis of 2D INADEQUATE spectra. Bistramide A demonstrated in vitro cytotoxicity towards human colon tumor HCT-116 and murine leukemia L1210 cell lines with an IC50 of 0.1 µg/mL.

Carbon-13 Chemical Shift Tensors in Aromatic Carbons. 4. Substituted Naphthalenes

Orendt, A.M.; Sethi, N.K.; Facelli, J.C.; Horton, W.J.; Pugmire, R.J. and Grant, D.M.
Journal of the American Chemistry Society, 1991 (in press). Funded by US Department of Energy and ACERC.

The principal values of the C-13 chemical shift tensor were measured for the aromatic carbons of 1,4,5,8-tetramethylnaphthalene, 1,2,3,6,7,8-hexadropyrene, and pyracene using the variable angle spinning technique. Ab initio calculations of the complete shielding tensors are also reported and provided the orientation of the principal values in the molecular frame. Good agreement between theory and experiment is obtained. The chemical shift principal values and orientations of these substituted naphthalenes correspond to those found in naphthalene; the observed differences can be rationalized in terms of the strain introduced by the alkyl substituent. In the naphthalenic derivatives studied, the bridgehead carbons exhibit relatively extensive pi-electron delocalization not found in some bridgeheads in more highly condensed aromatic systems.

Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 70:414-423, 1991. Funded by Pittsburgh Energy Technology Center and ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.

Carbon-13 Chemical Shift Tensors in Polycyclic Aromatic Compounds. 2. Single-Crystal Study of Naphthalene

Sherwood, M.H.; Facelli, J.C.; Alderman, D.W. and Grant, D.M.
Journal of the American Chemistry Society, 113:750-753, 1991. Funded by US Department of Energy.

Carbon-13 chemical shift tensors have been determined in single crystal nephthalene. The high field component of every tensor is oriented perpendicular to the molecular plane, as in other aromatic compounds. The bridgehead carbon tensor is nearly axially symmetric, with its low field component approximately perpendicular to the central bridging bond, in agreement with theoretical predictions. Protonated carbon tensors in naphalene are more asymmetric, with their low-field components directed approximately along the C-H bonds. The Pople model of chemical shielding with MNDO wave functions reproduces the experimental in-plane shielding components. These calculations show that the in-plane components of the carbon shift tensors are governed by the bond orders of the adjacent bonds, and shift tensors thus provide valuable information relating to aromaticity. The measured tensors deviate significantly from the symmetry of an isolated naphthalene molecule because of the lower symmetry of the molecular site in the crystal.

Predicting Vapor Pressures of Tar and Metaplast During Coal Pyrolysis

Fletcher, T.H.; Grant, D.M. and Pugmire, R.J.
ACS Division of Fuel Chem. Preprints, 36(1):250-257, 1991 (201st ACS National Meeting, Atlanta, GA, April 1991). Funded by Pittsburgh Energy Technology Center and ACERC.

Models of coal pyrolysis have progressed from simple one or two step empirical Arrhenius expressions that correlate total mass release during devolatilization, as reviewed by Anthony and Howard, to detailed descriptions of hydrocarbon chemistry and mass transport. These models describe the yields and compositions of pyrolysis products from coal under a wide range of heating conditions and ambient pressures. During pyrolysis of softening coals, a liquid phase appears that is referred to as metaplast. Release of pyrolysis gases and tar vapors inside the particle cause bubble formation in the softened coal particle, followed by swelling (increase in the particle diameter) with large internal voids (cenosphere formation). The softened state is followed by crosslinking or repolymerization which solidify the char matrix. As the coal particle is heated to sufficiently high temperatures, the light species in the metaplast are released as hydrocarbon vapors, along with light gases. Coal tar is generally defined to consist of those species which are released from the coal during pyrolysis which condense at room temperature and pressure. Low rank coals and lignite generally give low tar yields, and do not exhibit much softening or swelling behavior; this non-softening behavior may be caused by early crosslinking reactions. High rank coals (i.e., anthracites and low volatile bituminous coals) contain low amounts of volatile matter, and hence coal particles remain relatively intact during pyrolysis unless fragmentation occurs.

Mass transport affects coal pyrolysis in two ways: (1) as the ambient pressure increases, the tar yield decreases, and (2) as particle size increases, the tar yield decreases. However, there seem to be regions where the two mass transport effects are not controlling. For instance, in vacuum, the small pressure generated inside the pyrolyzing coal particle from the release of light gases and tar vapors may control the process. Also, total volatiles yields from a lignite were observed to remain constant with increasing ambient pressure, although this is probably due to the low tar yield of the lignite. Changes in coal pyrolysis yields as a function of particle size for diameters less than 200 µm are small.

Comparison of C-13 Chemical Shielding Anisotropy in Model Compounds and Coals with Theoretical Values

Pugmire, R.J.; Orendt, A.M.; Facelli, J.C. and Grant, D.M.
International Conference on Coal Science, University of Newcastle-upon-Tyne, U.K., 1991. Funded by US Department of Energy and ACERC.

The measurement of C-13 chemical shift tensor components in organic compounds has been shown to be valuable in gaining insight into the relationship between structure and the individual shift components, in a manner similar to earlier work on correlations between structural features and average or solution chemical shifts. In this work, the tensor components are measured for the aromatic carbons in several polycyclic aromatic hydrocarbons (PAHs); the compounds were chosen due to their incorporation of structural features that are believed to be important in the aromatic clusters in coals. The aromatic carbons in these compounds, as in coals can be divided into four groups: protonated, alkyl substituted, oxygen substituted, and bridgehead or condensed.

While there have been many solid state C-13 NMR measurements on both coals and PAHs using the anisotropy averaging technique of magic angle spinning (MAS), which measured only the average or isotropic chemical shift values, only a limited amount of data exists on the tensor components on either group of samples. For aromatic carbons, the majority of available data is on substituted benzenes; the only polycyclic aromatic compounds for which tensorial data exists in the literature are naphthalene (I), pyrene (V), coronene, and most recently buckminsterfullerene (C60). Therefore, before any correlations between the tensorial components and structure can be explored, much more data on this class of compounds must be obtained. Recently, the shielding data on 1,4,5,8-tetramethynaphthalene (II), 1,2,3,6,7,8-hexahydropyrene (III), pyracene (IV) (the alpha protonated carbons only), and triphenylene (VII) have been obtained on our laboratory, and the results are discussed below.

The chemical shielding tensor is a measure of the effect that the electronic environment has on changing the strength of the magnetic field experienced at the nucleus. Depending on the orientation a given molecule has with respect to the applied magnetic field, the observed resonance frequency changes. Therefore, the chemical shielding tensor is a reflection of the three dimensional electronic environment of the nucleus and, as such, it contains much potential information about structure and bonding. If one studies a powder sample, (where all possible orientations of the molecule can exist), a broad pattern is obtained from which the three principal values of the shielding tensor can be extracted. Each of the four classes of aromatic carbons has distinctive lineshape reflective of the differences in their electronic surroundings.

Experimentally, there are a number of methods that can be applied to obtain these tensor components. For the results presented below, measurement was either made on a static sample or the technique of variable angle sample spinning (VASS) was used. In the case of phenanthrene, isotopic C-13 labeling was used in the positions 1, 4, and 9, in order to obtain the tensor data on the three protonated carbons with very similar shielding tensors. Quantum mechanical calculations of the complete shielding tensor were also completed on these and other aromatic molecules of interest. These calculations are essential in providing the orientations of the tensor components in the molecular frame, information not obtained experimentally. Past experience has indicated that the calculated orientations are quite reliable (e.g. within ±7 º) in the cases where orientations have been determined experimentally either by single crystal or dipolar spectroscopy.

Measurement of C-13 Chemical Shielding Anisotropy in Coal

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Solid Carbonaceous Fuels, ACS Symposium Series, (R.E. Botto and Y. Sanada, eds.), 1991 (in press). Funded by US Department of Energy and ACERC.

The methods available in nuclear magnetic resonance to obtain the principal values of the shielding tensor are discussed. Applications to coal and to compounds that model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical shielding powder patterns is compared to results obtained by cross polarization with magic angle spinning (CP/MAS) and dipolar dephasing (DD).

1990

C-13 NMR Techniques for Structural Studies of Coals and Coal Chars

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Advances in Coal Spectroscopy, Meuzelaar, H.L.C., Editor, Plenum Publishing Corp., New York, 1990 (In press). Funded by ACERC and US Department of Energy.

Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.

Chemical Percolation Model for Devolatilization: 2. Temperature and Heating Rate Effects on Product Yields

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Energy & Fuels, 4 (54), 1990. Funded by Pittsburgh Energy Technology Center, US Department of Energy, National Science Foundation and ACERC.

The chemical percolation devolatilization (CPD) model previously developed to describe the devolatilization behavior of rapidly heated coal was based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe generation of finite tar clusters as labile bonds are cleaved in the infinite coal lattice. The model is used here to describe effects of heating rate and temperature on tar and gas release from coal. Coefficients for the net rate of competition between char formation and side-chain formation are generated from heated screen data performed at five different heating rates. The model also compares well with heated screen data obtained at 1000 K/s and different hold times at the final temperature as well as with data from entrained-flow reactors obtained at higher heating rates (104 K/s) where particle temperatures have been measured. Results indicate that the CPD model predictions yield good agreement with published data for a wide range of coals and particle heating rates.

Computerized Analysis of 2D INADEQUATE Spectra

Dunkel, R.; Mayne, C.L.; Curtis, J.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 90, 290-302, 1990. Funded by ACERC and US Department of Energy.

The extraction of carbon-carbon bond information from two-dimensional INADEQUATE spectra is both time consuming and complex due to the low sensitivity of the method, the incomplete suppression of single-quantum signals, and the large size of the data sets. A computerized analysis technique is introduced which detects bonds through a nonlinear regression analysis of carefully chosen subsets of the spectral data. A quantitative one-dimensional carbon spectrum is used to establish initial values for the regression and to determine the data subsets to be used. Using statistical analysis techniques, bonds are detected with reliability and sensitivity comparable to those of careful manual interpretation.

Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 1990 (In press). Funded by ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.

A Comparison of the IGLO and LORG Methods for the Calculation of Nuclear Magnetic Shieldings

Facelli, J.C.; Grant, D.M.; Bouman, T.D. and Hansen, A.E.
Journal of Computational Chemistry, 11 (32), 1990. Funded by National Institutes of Health, US Department of Energy, National Science Foundation and the Carlsberg Foundation.

The individual gauge for localized orbitals (IGLO) and localized orbital/local origin (LORG) methods for the calculation of chemical shieldings are compared from their theoretical and computational viewpoints. A detailed analysis of the fluorine substituent effect in a series of fluoromethanes is given in terms of the IGLO and LORG bond contributions. The performance of both methods is discussed for molecular systems of fairly different sizes.

Two-Dimensional Chemical-Shift Tensor Correlation Spectroscopy. Analysis of Sensitivity and Optimal Measurement Directions

Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Journal of Magnetic Resonance, 86, 60-69, 1990. Funded by US Department of Energy.

Chemical-shift tensors can be determined from a single crystal placed in six or more orientations in a magnetic field. The sensitivity of this determination as a function of the selected crystal orientations is considered with a statistical figure of merit. A variety of configurations are examined, and it is found that the set of six orientations based on the vertices of an icosahedron optimizes the figure of merit and results in the most sensitive determination of the tensor. The relationship of these optimal orientations to those used previously in two-dimensional chemical-shift tensor correlation spectroscopy is discussed. It is shown that the high symmetry of the icosahedron simplifies the design on which the construction of a new sample orienting mechanism may be based.

Rotational Resonance in a Spin-Lock Field for Solid State NMR

Gan, Z.H. and Grant, D.M.
Chemical Physics Letters, 168 (3,4), 1990. Funded by a National Institute of Health research grant from the Institution of General Medical Science.

In the magic angle spinning NMR experiment, a rotational resonance phenomenon occurs whenever a spin-lock rf field is either equal to or twice the sample spinning speed. The resonance occurs in the rotating frame where the spin-lock field assumes the role of the "Zeeman" interaction and the chemical shift modulated by the sample rotation is the "radio frequency" irradiation. The importance of good rf field homogeneity to the experiment is shown.

NMR Spectra from Powdered Solids Spinning at Any Angle and Speed: Simulations and Experiments

Sethi, N.K.; Alderman, D.W. and Grant, D.M.
Molecular Physics, 71 (2), 217-238, 1990. Funded by US Department of Energy.

A new method is described for calculating NMR spectra of powdered solids spinning at any speed and at any angle relative to the magnetic field. The algorithm provides the intensities and shapes of the center band and all sidebands through a computation involving the solution of homogeneous linear equations. The method is also applicable to cases where two or more inter-actions simultaneously affect the spectral response. A simple extension of the algorithm treats cases where an analytical expression for the resonance frequency is not available. The method is demonstrated with simulations and experimental examples of anisotropic chemical-shift spectra obtained by spinning samples at various low speeds and angles. The technique is also applied to a spin-1/2 nucleus with both an anisotropic chemical shift and a dipolar coupling either to a heteronuclear spin-1/2 nucleus or to a spin-1 nucleus experiencing a strong quadrupole interaction.

Molecular and Structural Information from Variable-Angle Spinning NMR Dipolar Spectra of C-13/N-14 Systems

Gan, Z.H. and Grant, D.M.
Journal of Magnetic Resonance, 1990 (In press). Funded by National Institute of Health.

A sample-spinning solid-state NMR study of a spin-1/2 nucleus coupled to a quadrupolar nucleus is presented. Using a simple approach, the quadrupolar effect is expanded in terms of irreducible spherical tensor components up to / = 4. Information on the electric field gradient tensor orientation, dipolar coupling, and chemical-shielding tensor for the C-13/N-14 system can be obtained experimentally from the magic-angle and the off-magic-angle sample-spinning spectra. Satisfactory results are obtained by comparing the simulations with the experimental spectra of tetramethylpyrazine, dimethyglyoxime, and triethylenediamine.

Carbon-13 Chemical Shift Tensors in Polycyclic Aromatic Compounds. 2. Single-Crystal Study of Naphthalene

Sherwood, M.H.; Facelli, J.C.; Alderman, D.W. and Grant, D.M.
Am. Chem. Soc., 1990 (In press). Funded by US Department of Energy.

Carbon-13 chemical shift tensors have been determined in single crystal nephthalene. The high field component of every tensor is oriented perpendicular to the molecular plane, as in other aromatic compounds. The bridgehead carbon tensor is nearly axially symmetric, with its low field component approximately perpendicular to the central bridging bond, in agreement with theoretical predictions. Protonated carbon tensors in naphalene are more asymmetric, with their low-field components directed approximately along the C-H bonds. The Pople model of chemical shielding with MNDO wave functions reproduces the experimental in-plane shielding components. These calculations show that the in-plane components of the carbon shift tensors are governed by the bond orders of the adjacent bonds, and shift tensors thus provide valuable information relating to aromaticity. The measured tensors deviate significantly from the symmetry of an isolated naphthalene molecule because of the lower symmetry of the molecular site in the crystal.

A Chemical Percolation Model of Coal Devolatilization: 3. Direct Use of C-13 NMR Data to Predict Effects of Coal Type

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J.; Solum, M.S. and Grant, D.M.
Polycyclic Aromatic Compounds, 1:251-264, 1990. Funded by Gas Research Institute and ACERC.

The chemical percolation devolatilization (CPD) model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe the generation of tar precursors of finite size based on the number of cleaved labile bonds in the infinite coal lattice. The chemical percolation devolatilization model described here includes treatment of vapor-liquid equilibrium and a cross-linking mechanism. The cross-linking mechanism permits reattachment of metaplast to the infinite char matrix. A generalized vapor pressure correlation for high molecular weight hydrocarbons, such as coal tar, is proposed based on data from coal liquids. Coal-independent kinetic parameters are employed. Coal-dependent chemical structure coefficients for the CPD model are taken directly from C-13 NMR measurements, with the exception of one empirical parameter representing the population of char bridges in the parent coal. This is in contrast to the previous and common practice of adjusting input coefficients to precisely match measured tar and total volatiles yields. The CPD model successfully predicts the effects of pressure on tar and total volatiles yields observed in heated grid experiments for both bituminous coal and for lignite. Predicted tar molecular weights are consistent with size-exclusion chromatography (SEC) data and field ionization mass spectrometry (FIMS) data. Predictions of average molecular weights of aromatic clusters as a function of coal type agree with corresponding data from NMR analyses of parent coals. The direct use of chemical structure data as a function of coal type helps justify the model on a mechanistic rather than an empirical basis.

Measurement of C-13 Chemical Shielding Anisotropy in Coal

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Solid Carbonaceous Fuels, ACS Symposium Series, Botto, R.E. and Sanada, Y., Editors, 1990 (In press). Funded by ACERC and US Department of Energy.

The methods available in nuclear magnetic resonance to obtain the principal values of the shielding tensor are discussed. Applications to coal and to compounds that model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical shielding powder patterns is compared to results obtained by cross polarization with magic angle spinning (CP/MAS) and dipolar dephasing (DD).

Solid-State C-13 and H-1 NMR Studies of the Evolution of the Chemical Structure of Coal Char and Tar During Devolatilization

Fletcher, T.H.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Pugmire, R.J.
Twenty-third Symposium (International) on Combustion, The Combustion Institute, France, 1990 (In press). Funded by US Department of Energy and ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the chemical structure of char particles and condensed tar vapors produced as pyrolysis products from an Illinois #6 coal at rapid heating conditions (~104 K/s) at two gas conditions (maximum gas temperatures of 1250 K and 1050 K). The temperature history of particles in the flow reactor is determined using a unique infrared sizing-pyrometry system. The C-13 NMR analyses of the coal chars indicate that significant amounts of aliphatic material are released from the coal during devolatilization, with little change to the aromatic cluster size or number of attachments per cluster. At long residence times, and at higher temperatures, small increases in the cluster size in the char are observed. The H-1 NMR analyses indicate that thermal decomposition of tar vapor occurs at the 1250 K gas condition, as evidenced by increases in the aromaticity and decreases in the peripheral aliphatic groups, such as methyl groups and aliphatic bridge material.

1989

Use of Carbon-13 Nuclear Magnetic Resonance in Conformational Analysis of Hydroaromatic Compounds, in Conformational Analysis of Cyclohexenes, Cyclohexadienes, and Related Hydroaromatic Compounds

Morin, F.G. and Grant, D.M.
VCH Publishers, New York, Peter W. Rabideau (Editor), 1989 (In press). Funded by National Institutes of Health.

Since the original observation or nuclear magnetic resonance (NMR), the technique of solution-state NMR has become a premier method of structural and conformational analysis of organic molecules. With the introduction of Fourier transform methods, C-13-NMR has exceeded H-1-NMR as a tool for conformational analysis and a number of books have become standard reference texts in this field.

The chemical shift experienced by a C-13 nucleus is determined by several electronic factors such as hybridization and inductive effects of substituents. Relatively smaller but highly informative changes in chemical shift result from even subtle changes in conformation, for example, the difference in shift of an equatorial and an axial methyl group in methylcyclohexane. The utility of conformationally dependent C-13 shifts in hydroaromatic compounds is the focus of this chapter.

Hydroaromatic compounds such as terralin (1,2,3,4-tetrahydronaphthalene), while of significant importance in the field of fuel science as reactive components in the liquefaction and gasification of coals, have not received their share of NMR attention over the years compared with the well-studied and understood saturated ring systems such as cyclohexane. These mixed cyclic systems (both aromatic and aliphatic) possess interesting differences compared with either the pure aliphatic or aromatic ring systems. Therefore, a good understanding of the conformational properties and the energetics involved in these compounds would be valuable. An organized and systematic study of the conformationally dependent C-13 chemical shifts of these molecules is aided by the use of a multiparameter regression analysis. To introduce this technique and to provide a comparison of the conformational properties of the cyclohexanes with hydroaromatics, we begin with a brief historical review of the use of C-13 chemical shifts in the conformational analysis of alkanes and cyclohexanes.

Molecular Structure and Carbon-13 Shielding Tensors Obtained From Nuclear Magnetic Resonance

Facelli, J.C. and Grant, D.M.
Book Chapter to be published in Vol. 9 of Topics in Stereochemistry, E. L. Eliel and S. H. Wilen (Eds.), 1989. Funded by US Department of Energy, National Institutes of Health, and National Science Foundation.

One of the most important observations in the early days of NMR spectroscopy was the dependence of the magnetic resonance frequency upon the chemical or electronic environment of the nucleus. These experimental results were followed rather quickly by Ramsey's theoretical formulation (1) of chemical shielding which recognized the tensorial character of the interaction. The physical basis of chemical shielding may be visualized from an idealized experiment in which the transition frequency of a nuclear spin is considered for an isolated molecule at various orientations relative to an external magnetic field. The local magnetic field at the nucleus depends upon the orientation of the molecule with respect to the external magnetic field as a consequence of differential shielding resulting from anisotropy in the electron distribution in the vicinity of the nucleus. Thus, the reorientation of such an idealized molecule would be accompanied by a shift in the NMR frequency of the nucleus. A similar but realizable experiment, to be discussed later, can be performed by reorienting a single crystal in a magnetic field while following the orientational dependence of the resonance frequency. These data determine the C-13 shielding tensors and provide insight into the three dimensional distribution of molecular electrons.

For many years, owing to experimental limitations, NMR spectra of organic compounds were studied to any appreciable extent only in the liquid phase, where the rapid molecular tumbling prevented the observation of the tensorial features of chemical shielding. Only average or 'isotropic' shifts can be determined from liquid phase data. Even so, early isotropic C-13 chemical shifts were found to depend significantly upon molecular conformations. The importance of conformational effects found in the cyclic alkanes firmly established C-13 resonance methods as a powerful tool in the field of conformational analysis.

When one records only the isotropic chemical shifts obtained from liquids, a great deal of information on the shielding tensor is left unmeasured, Molecular rotation, rapid on the NMR time scale, averages the components of the shielding tensor to its isotropic value and eliminated the three-dimensional information available in the full tensor. Furthermore, quenching of the molecular motion in the solid state often provides interesting additional information which is absent in both liquids and gases. The immobilization of molecules in a solid is usually associated with a freezing of the conformational structure as observed in a number of solid samples. The rigidity imposed by the crystal lattice may destroy the effective symmetry found in rapidly equilibrating molecules. Such a break in symmetry can give rise to more lines in a magic angle spinning (MASS) spectrum than are found in the corresponding liquid phase spectrum. A brief description of some relevant results, obtained using cross polarization (CP) and MAS methods in organic molecules is given as a background for the results obtained from tensor information. CP/MAS can not only be used to understand conformational changes in solids, but at times becomes the best tool for conformational analysis of materials (e.g. amorphous substances) that are unsuitable for X-ray studies. The CP/MAS method is also useful for relatively insoluble materials (e.g. intractable polymers) or molecules for which conformational information cannot be obtained by NMR in solution. It is apparent that a great deal of information on molecular electronic structure can be obtained if the C-13 chemical shift tensors, or at least their principal values, are measured and properly rationalized. These techniques have been applied to several types of solids including single crystals, polycrystalline and amorphous powders, frozen liquids and gases, and matrix isolated compounds, using rather elaborate instrumental techniques, complicated data reduction schemes, and extensive theoretical methods to correlate the molecular structural information with the experimental results. Isolation of organic compounds at cryogenic temperatures in glasses and matrices deserves special comment because in many cases it is the only accessible method for studying highly reactive compounds using NMR.

C-13 Solid-State NMR of Argonne Premium Coals

Solum, M.S.; Pugmire, R.J. and Grant, D.M.
Energy & Fuels, 3, 187, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Eight Argonne Premium Coal samples and three other oxidized coals have been investigated by the C-13 CP/MAS NMR technique. Spin-lattice relaxation, variable contact time, and dipolar-dephasing experiments were performed on each coal. The proton spin-lattice relaxation time, T1H, the proton spin-lattice relaxation in the rotating frame time, T1pH, and the cross-polarization time, TCH, are reported along with the dipolar-dephasing time constants, TG, and TL, for both the aromatic and aliphatic regions of the 11 coals. These data, together with normal CP/MAS integrations over selected chemical shift ranges, have been used to derive 12 parameters relating to the carbon skeletal structure, including the aromaticity. From the mole fraction of bridgehead or inner carbons as measured by NMR, the aromatic cluster size may be estimated for each coal.

Chemical Percolation Model for Devolatilization: II. Temperature and Heating Rate Effects on Product Yields

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Accepted for publication by Energy & Fuels, 1989. Funded by US Department of Energy and ACERC (National Science Foundation and Associates and Affiliates).

The CPD model previously developed to describe the devolatilization behavior of rapidly heated coal was based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe generation of finite tar clusters as labile bonds are cleaved in the infinite coal lattice. The model is used here to describe effects of heating rate and temperature on tar and gas release from coal. Coefficients for the net rate of competition between char formation and side chain formation are generated from heated screen data performed at five different heating rates. The model also compares well with heated screen data obtained at 1000 K/s and different hold times at the final temperature as well as with data from entrained flow reactors obtained at higher heating rates (104 K/s) where particle temperatures have been measured. Results indicate that the CPD model predictions yield good agreement with published data for a wide range of coals and particle heating rates.

Polyethylene Crystallinity From Static, Solid-State NMR Spectra

Hughes, C.D.; Sethi, N.K.; Baltisberger, J.H. and Grant, D.M.
Macromolecules, 22, 2551, 1989. Funded by US Department of Energy.

A new method for determining the crystallinity of linear polyethylene (LPE) utilizing static, solid state C-13 Nuclear Magnetic Resonance (NMR) spectroscopy and computer lineshape simulation is described. Good agreement between the crystallinity measured by this method and a standard method was found. The computer simulation of the experimental spectrum also reflects the degree of gauche and trans chain conformations in the amorphous region of the sample.

Two-Dimensional Chemical Shift Tensor Correlation Spectroscopy. Analysis of Sensitivity and Optimal Measurement Directions

Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Accepted for publication in J. Mag. Res., 1989. Funded by US Department of Energy.

Chemical shift tensors can be determined from a single crystal placed in six or more orientations in a magnetic field. The sensitivity of this determination as a function of the selected crystal orientations is considered with a statistical figure of merit. A variety of configurations are examined, and it is found that the set of six orientations based on the vertices of an icosahedron optimizes the figure of merit and results in the most sensitive determination of the tensor. The relationship of these optimal orientations to those used previously in two-dimensional chemical shift tensor correlation spectroscopy is discussed. It is shown that the high symmetry of the icosahedron simplifies the design on which the construction of a new sample orienting mechanism may be based.

Distortion in Six-Membered Saturated Rings by Natural Abundance Deuterium NMR

Curtis, J.; Grant, D.M. and Pugmire, R.J.
J. Am. Chem. Soc., 111, 7711, 1989. Funded by National Institutes of Health.

Distortions in six-membered aliphatic ring systems were examined using deuterium chemical shifts and multiple linear regression analysis. A new set of regression parameters has been developed that successfully interprets not only spectra for those methylcyclohexanes with normal chair conformations but also spectra for cis- and trans-decalin and for three highly distorted trimethylcyclohexanes with skew-boat type distortions to their equilibrium conformations. The origin of deformations in the cyclohexane ring and the effect of distortions on ring hydrogen/deuterium chemical shifts are discussed in terms of gauche interactions between vicinal C-C and C-D bonds. The fits for 115 deuterium shifts in these cyclic alkanes exhibited a multiple R = 0.9816 and s = 0.0623 ppm for an overall shift range of about 1.5 ppm. As the 14 structural parameters obtained in this study are very sensitive to distortions and conformational features, the results indicate that ²H NMR methods should be very powerful in those cyclic paraffins in which highly second-order banding is observed in the corresponding H-1 NMR spectrum.

Magnetization Preparation for Coupled Relaxation Studies Using J-Spectral Pulse Sequences (Technical Note)

Liu, F.; Mayne, C.L. and Grant, D.M.
J. Mag. Res., 84, 344, 1989. Funded by National Institutes of Health.

The accuracy with which one can determine spectral densities in a coupled relaxation experiment is strongly dependent on the nature of the non-equilibrium states from which the spin system is allowed to relax. Pulse sequences similar to those used to obtain 2D J-spectra have been used to prepare initial non-equilibrium states of an AX2 spin system. These preparations create a maximum perturbation for one of the magnetization modes which was poorly perturbed by other spin preparations. Inclusion of data obtained using the new preparation significantly reduces the marginal standard deviations of several of the spectral densities and reduces correlations among the spectral densities. Thus, a significant improvement in the structural and dynamical parameters extracted from the spectral densities is achieved.

Solid-State C-13 NMR Studies of Coal Char Structure Evolution

Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Fletcher, T.H. and Solomon, P.R.
Fuel Fiv. Preprints, 34 (4), 1337-1346, 198th ACS National Meeting, Miami, 1989. (Also presented at the Western States Section, The Combustion Institute Spring Meeting, Pullman, Washington, 1989.) Funded by ACERC (National Science Foundation and Associates and Affiliates).

Solid state C-13 NMR techniques have been used to study the evolution of char structure during pyrolysis processes. The effects of residence time, heating rate, and final char temperature are observed. The NMR data demonstrates that extensive loss of aromatic ring bridge material precedes significant change in aromatic cluster size.

Improvement of Double-Tuned Probe Circuit for High Field Solid-State NMR

Jiang, Y.J.; Woolfenden, W.R.; Orendt, A.M.; Anderson-Altmann, K.L.; Pugmire, R.J. and Grant, D.M.
30th Experimental NMR Conference, Pacific Grove, California, 1989. Funded by US Department of Energy.

A double tuned probe circuit used in high field solid-state NMR experiments has been evaluated. The idea of improved efficiency in both the decoupling and observe channels has been considered, with emphasis on alleviating the stray capacitance in the proton channel. The results of solid-state NMR experiments are included.

Computerized Analysis of 2D-Inadequate Spectra

Dunkel, R.; Mayne, C.L.; Pugmire, R.J. and Grant, D.M.
30th Experimental NMR Conference, Pacific Grove, California, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

A phase sensitive two-dimensional INADEQUATE spectrum contains carbon-carbon connectivity information; and, hence, allows one to trace molecular carbon skeletons. When a bond is present between two carbons, there are antiphase doublets centered at the chemical shift of each of the carbons. This AB pattern is displaced in F1 by the double quantum frequency that is the sum of the chemical shifts. For each pair of lines in the 1D spectrum two small regions may be defined which must contain the pattern if a bond is present. Using eight parameters the AB pattern can be modeled as a sum of four two dimensional Lorentzians. A nonlinear least squares fit of this pattern to the experimental data from the two small regions is then performed for each pair of resonances in the 1D spectrum. The best-fit values of the parameters and the covariance matrix are used to decide whether the two carbons are bonded.

The Use of Pattern Recognition With High Resolution H-1 and C-13 NMR Techniques in the Study of Fuels and Coal-Derived Liquids

Pugmire, R.J.; Dunkel, R.; Mayne, C.L. and Grant, D.M.
1989 International Conference on Coal Science, Vol. 2,Tokyo,1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

NMR spectroscopy is a widely used analytical tool for probing molecular structure and dynamics. The growth in the use of this tool for studying coal-derived liquids has been quite dramatic. Spectral analysis of these complex hydrocarbon mixtures, however, has been hampered by the vast amount of data present in any given spectrum. We have embarked on a program to use multiple-pulse, two-dimensional (2D) NMR spectroscopy (1, 2) coupled with computer assisted analysis techniques to extract structural information from these materials. Described herein is a program called CCBOND for the computer aided analysis of 2D-INADEQUATE spectra.(3)

A Chemical Percolation Model for Devolatilization: Temperature and Heating Rate Effects

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Fuel Div. Preprint, 34 (4), 1272-1279, 198th ACS National Meeting, Miami, 1989. Funded by US Department of Energy and ACERC (National Science Foundation and Associates and Affiliates).

It is well known that the yield of volatile matter obtained from a pulverized coal is dependent upon the temperature history of the particle. However, the effect of heating rate on volatiles yield is difficult to study independently of final temperature. For example, the volatile yields obtained in an entrained flow reactor study by Kobayashi, et al. increase with both temperature and heating rate, but the independent contribution of heating rate could not be assessed. Heated screen experiments were developed to study devolatilization behavior at different heating rates independently from the final particle temperature. The data of Anthony and Howard show little increase in volatiles yield when particles are heated to the same final temperature on a heated screen at different heating rates. In a more recent study, Gibbins-Matham and Kandiyoti show evidence for small increases in the volatiles yield from a Pittsburgh #8 coal as the heating rate is increased from 1 K/s to 1000 K/s on a heated screen. Coal samples were heated at 5 different heating rates to a final temperature of 700ºC and held for 30 s. Experiments were repeated several times in order to ensure accuracy of the data. The total volatiles yield increases from 41.5% at 1 K/s to 46.8% at 1000 K/s, a relative increase in yield of 13%. This increase in yield with increase in heating rate is small, but is larger than associated experimental errors.

The chemical percolation devolatilization (CPD) model was developed as a means to describe coal devolatilization behavior based upon the chemical structure of the patent coal. Some of the input parameters for this model are obtained from NMR characterizations of the parent coal. Percolation statistics are used to describe the probability of generating finite tar fragments from the infinite coal matrix. Pyrolysis yields of tar, gas, and char for three different types of coal are described using a single set of kinetic parameters: only chemical structure parameters are changed for the different coals. The initial description of the CPD model allowed for a temperature dependence of the competition between side chain formation and char formation. However, this option was not exercised in the initial study in order to demonstrate general utility of the model for one set of devolatilization data on three coals collected over a narrow range of temperatures and heating rates. In the present work, the Gibbins-Matham and Kandiyoti data are used to determine additional coefficients for the CPD model that accurately predict the changes in char and tar yield as a function of heating rate.

1988-1987

C-13 Chemical Shielding Anisotropy Studied by Variable-Angle Sample Spinning

Sethi, N.K.; Grant, D.M. and Pugmire, R.J.
Journal of Magnetic Resonance, 71, 476-479, 1987. 4 pgs. Funded by US Department of Energy.

Spinning the sample at angles other than the magic angle is shown to have interesting applications for obtaining chemical shielding anisotropies for systems where severe overlapping makes the analysis of static powder pattern either cumbersome or impossible. Results for 1,3,5-trimethoxybenzene and p-dimethoxybenzene are in excellent agreement with those obtained via single-crystal studies.

Structural Variations and Evidence of Segmental Motion in the Aliphatic Region in Coals Observed with Dipolar-Dephasing NMR

Soderquist, A.; Burton, D.J.; Pugmire, R.J.; Beeler, A.J.; Grant, D.M.; Durand, B. and Huk, A.Y.
Energy and Fuels, 1, (1), 50-55, 1987. 6 pgs. Funded by US Department of Energy and Standard Oil Co.

We report the structural variations of a number of coals and coal macerals. Through the time constants associated with dipolar-dephasing techniques, CP/MAS spectral data reveal the presence of segmental motion in certain low-rank coal samples. The motion detailed is in the aliphatic region and is thought to be due to CH2 groups associated with hydroaromatic and/or polymethylene structural units.

An Efficient Double-Tuned C-13/H-1 Probe Circuit for CP/MAS NMR and Its Importance in Linewidths

Jiang, Y.J.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 71, 485-494, 1987. 10 pgs. Funded by Standard Oil, US Department of Energy, Office of Energy Research and Basic Energy Sciences.

A simple double-tuned C-13/H-1 probe for high-field CP/MAS NMR is described. The tuning and matching can be adjusted separately in each channel and made exactly equal to 50 W. Isolation between H-1 and C-13 channels exceeds 40 dB. The sensitivity (S/N) of the observation channel (C-13) for methyl carbons is close to 47 in a CP/MAS experiment of 10 scans on 70 mg of a sample of hexamethylbenzene for the rotor sample volume of 140 ml. The probe delivers a 2.4 ms 90º proton pulse with only 90 W at 200 NHz and 250 W at 50.3 MHz. Examples are presented of limewidth reduction with higher decoupling power. The effects of anisotropy in the bulk magnetic susceptibility are also discussed.

Efficient Stator/Rotor Assembly for Magic-Angle Spinning NMR

Jiang, Y.J.; Woolfenden, W.R.; Alderman, D.W.; Mayne, C.L.; Pugmire, R.J. and Grant, D.M.
Rev. SCI Instrum, 58, (5), 755-758, 1987. 4 pgs. Funded by US Department of Energy and Office of Basic Energy Sciences.

A newly designed stator assembly for cylindrical spinners used in magic-angle spinning nuclear-magnetic resonance experiments is described. Separate driving and bearing gas chambers allow variable and stable spinning speeds, and this design permits easy starting and stopping of the rotor. Isolation of the chambers is achieved with the application of pressure screws rather than O-rings or glue lines to avoid leakage at high gas pressures. The overall dimensions are optimal to facilitate easy assembly. Some significant modifications have been made to an earlier spinner design. These improvements give better efficiency and concentricity of the spinner. Applications are illustrated with carbon-13 cross polarization/magic angle spinning (CP/MAS) spectra carried out at different rotor spinning rates.

Correlation of Ring Nitrogen Substituents with Carbon-13 Nuclear Magnetic Resonance Data in Azoloazines

Pugmire, R.J.; Smith, J.C.; Grant, D.M.; Stanovnik, B.; Tisler, M. and Vercek, B.
J. Heterocyclic Chem, 24, 805-809, 1987. 5 pgs. Funded by Institute of General Medical Sciences of the National Institutes of Health and by the Council of Yugoslav Academies, the Slovene Academy of Science and Arts, and the US National Academy of Sciences.

Carbon-13 nuclear magnetic resonance data have been acquired on 22 azoloazines. Chemical shifts have been correlated by a step-wise linear multiple regression with nitrogen substituents in both the 5- and 6- membered rings using pyrrolo [1,2-a] pyridine as the reference for chemical shift correlation. The data demonstrate that a highly correlated set of chemical shift parameters exist. Nitrogen substitution in the five-membered ring produces larger cross-ring effects than is observed in the five-membered ring when substitution occurs in the six-membered ring. Within the six-membered ring a constant para-substituent parameter is noted. The meta- and para- parameters are more complex and fall into two groups for each parameter. Within the five-membered ring, a highly regular chemical shift pattern is observed which reflects an attenuated perturbation from nitrogen substitution in the six-membered ring.

Quantitative Determination of Different Carbon Types in Fusinite and Anthracite Coals From Carbon-13 Nuclear Magnetic Resonance Chemical Shielding Line-Shape Analysis

Sethi, N.K.; Pugmire, R.J.; Facelli, J.C. and Grant, D.M.
Analytical Chemistry, 60, 1574-1606, 1988. 32 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

C-13 NMR shielding tensors have been determined for two anthracite coals and a fusinite maceral using powder pattern line-shapes that have been analyzed as a superposition of three different bands due to benzene-like, condensed (bridgehead and inner) and substituted carbons. Theoretical calculations on circumcoronene (I) as a model compound support the interpretations of the experimental data. Determination of the ratio of non-protonated to protonated aromatic carbons obtained on the anthracites by the spectroscopic analysis is in excellent agreement with the elemental analysis and previous studies by dipolar dephasing NMR techniques. The method therefore constitutes a valuable way to analyze the structure of high rank coals and should be useful in char characterization. The mole fraction of condensed carbons obtained by this technique is used to estimate the average cluster size in these polycondensed aromatic hydrocarbon materials.

Low-Temperature C-13 Magnetic Resonance 8. Chemical Shielding Anisotropy of Olefinic Carbons

Orendt, A.M.; Facelli, J.C.; Beeler, A.J.; Reuter, K.; Horton, W.J.; Cutts, P.; Grant, D.M. and Michl, J.
American Chemical Society, 110, 3386, 1988. Funded by US Department of Energy.

The principal values of the C-13 NMR shielding tensor were measured at cryogenic temperatures for a series of olefinic carbons, including methyl-substituted ethylenes, 1-methyl- and 1,2-dimethylcycloalkenes, methylenecycloalkanes, and bicyclo (n,m.0) alkenes. Information on the orientation of the principal axes was obtained from ab initio calculations of the chemical shielding tensor using the IGLO (individual gauge for localized orbitals) method. The results for several compounds with unusual principal values of the shielding tensor were analyzed in terms of the bond contributions in the principal axis system.

Carbon-13 Chemical Shift Tensors in Single-Crystal Methoxybenzenes

Carter, C.M.; Facelli, J.C.; Alderman, D.W.; Grant, D.M.; Dalley, N.K. and Wilson, B.E.
J. Chem. Soc. Faraday Trans. 1, 1988. Funded by US Department of Energy.

Using a two-dimensional NMR orientation correlation technique carbon-13 chemical-shift tensors have been measured in single crystals of 1.4-dimethoxybenzene. 1,3,5-trimethoxybenzene and 1,2,3-trimethosybenzene. The two-dimensional technique greatly extends the single-crystal method to materials with a much larger number of different carbon-13 resonance lines. The manner of dealing with chemically identical, but magnetically non-equivalent carbons in the unit cell is discussed. The X-ray structure of 1, 2, 3-trimethoxybenzene is reported for the first time, and a redetermination of the X-ray structure for 1.4-dimethoxybenzene is given. The principal values of all carbon-13 chemical-shift tensors in the three molecules and the orientation of their principal axes have been obtained. Using multiple regressional analysis the principal values of the tensors have been discussed in terms of additive substituent effects. AB initio calculations of the shielding tensors in anisole (methoxybenzene) and benzene were used to calculate substituent effects that agree closely with the parameters obtained from the regressional analysis.

Double Resonance Interferometry, Relaxation Modulated Coherence Transfer

Newmark, R.D.; Alderman, D.W. and Grant, D.M.
Submitted to Phys. Rev., 1988. Funded by US Department of Energy.

The double resonance interferometry experiment of Stoll, Vega and Vaughan is examined with numerical simulations employing the complete Redfield matrix. It is shown that their observed doublet asymmetry is a consequence of random field relaxation mechanisms. The relaxation pathways in the system are examined in detail to explain the coherence transfer that takes place during the SVV experiment. The simulations must include relaxation during the selective carbon-13 pulses in order to agree with the experimental results thereby indicating the need for care in using impulse approximations for selective pulses in some double resonance experiments.

The Effect of Transverse Cross Relaxation of NMR Dipolar Spectra

Gan, Z.H.; Facelli, J.C. and Grant, D.M.
Submitted to J. Chem. Phys., 1988. Funded by US Department of Energy.

The C-13 dipolar powder spectra of C-13H3P-31O(OH)2 at room temperature and C-13H3F-19 at low temperature (T=25K) are reported. An intense peak is observed in the center of the Pake doublet. This phenomenon is explained by the transverse cross relaxation mechanism between the C-13 and either the P-31 or F-19 nuclei. The chemical shielding tensors and dipolar coupling constants of these two molecules are obtained by fitting the experimental spectra.

Time-Domain Fitting and Director Reorientation of H-1-NMR Spectra of Molecules Dissolved in Liquid-Crystal Solvents

Heeschen, W.A.; Alderman, D.W. and Grant, D.M.
Submitted to J. Phys. Chem, 1988. Funded by US Department of Energy.

A time-domain fitting routine is described and applied to the analysis of a H-1-NMR spectrum of a solute in a liquid-crystalline solvent. Principles of the time-domain fitting technique are described in terms of data selection, baseline compensation and computational advantage over other fitting methods. Also, the use of fitting coefficients based on structural variation is described and found to provide rapid, reliable spectrum fits with physically reasonable structures. Solute ordering information is determined through interpretation of high-resolution spectra using director reorientation by the variable-angle sample spinning technique to provide a starting point for the fitting. Spectra from norbornadiene dissolved in the liquid crystal p-pentylphenyl 2-chloro-4-(p-pentylbenzoyloxy) benzoate are used to demonstrate the method. The resultant fit of the norbornadiene structure is compared to results obtained previously by other workers.

C-13 Dipolar Spectroscopy of Nitromethane

Solum, M.S.; Facelli, J.C.; Gan, Z.H. and Grant, D.M.
Submitted to Molecular Physics, 1988. Funded by US Department of Energy.

The C-13 low temperature (25K) dipolar spectrum of C-13H3N-14O2 is reported. The spectrum was fit and the principal values of the C-13 shielding tensor are given. The C-13 shielding tensor principal values are similar to those found in other C-13H3 groups. Also, the N-14 quadrupolar parameters and the direct dipolar couplings were obtained from the fit. Because of librational motions an effective trace term in the dipolar interaction has to be included in the fit.

Distortion in Six-Membered Saturated Rings by Natural Abundance Deuterium NMR

Curtis, J. and Grant, D.M.
Submitted to J. Am. Chem. Soc., 1988. Funded by US Department of Energy.

Distortions in six-membered aliphatic ring systems were examined using deuterium chemical shifts and multiple linear regression analysis. A new set of regression parameters has been developed which successfully interprets not only spectra for those methylcyclohexanes with normal chair conformations, but also spectra for cis- and trans-decalin and for three highly distorted trimethylcyclohexanes with skw boat type distortions to their equilibrium conformations. The origin of deformations in the cyclohexane ring and the effect of distortions on ring hydrogen/deuterium chemical shifts are discussed in terms of gauche interactions between vicinal C-C and C-D bonds. The fits for 115 deuterium shifts in these cyclic alkanes exhibited a multiple R-=0.9816 and s=0.0623 ppm for an overall shift range of about 1.5 PPM As the 14 structural parameters obtained in this study are very sensitive to distortions and conformational features, the results indicate that 2H NMR methods should be very powerful in those cyclic paraffins in which highly second-order banding is observed in the corresponding proton NMR spectrum.

Application of High Resolution Solid State NMR to Analysis of the Chemical Shift Anisotropy Tensors in Coals

Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
1987 International Conference of Coal Science, 1987, 41-44, Science, Holland, 1987. 4 pgs. Funded by US Department of Energy, Basic Energy Sciences, and ACERC (National Science Foundation and Associates and Affiliates).

Carbon-13 NMR spectroscopy has become an important tool in the structural determination of coal. The major techniques used are cross polarization magic angle spinning (CP/MAS) and dipolar dephasing (DD/MAS) experiments. The CP/MAS technique has been extensively used on both whole coals and macerals by this laboratory as a direct method for determining the fraction of aromatic carbons, fa, in these samples. The dipolar dephasing experiment (DD/MAS) provides additional definition of the structure of coal by using the C-13-H-1 dipolar coupling to separate the carbons into sub-classes, i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aromatic and aliphatic regions of the spectrum. The application of the technique to coals has been thoroughly described.

MAS experiments produce narrow lines in solids but, in doing so, valuable chemical shift anisotropy (CSA) information is lost as the chemical shift tensors are averaged to their isotropic values. CSA is a second rank tensor and is characterized by three unique resonance frequencies s11, s22 and s33 called the principal values of the tensor elements . Knowledge of all three frequencies in addition to the isotropic value (average of tensor elements) in a source of information which is indispensable for molecular characterization and independent peak assignments. Measurement of the tensor elements is carried out by recording the C-13 NMR spectrum of a finely powdered sample to obtain the solid state line shape which can then be analyzed to determine tensor elements of chemically and structurally different types of carbons. Other noteworthy techniques for extracting such data are analysis of spinning sidebands from slow magic angle spinning experiments and non-magic angle sample spinning referred to in this paper as variable angle sample spinning (VASS). In the present study, we have analyzed both static and variable angle sample spinning line shapes to obtain the CSA tensor values and population factors for the aromatic carbons of coals.

C-13 NMR Structural Determination of the Coals in the Premium Coal Sample Bank

Solum, M.S.; Pugmire, R.J. and Grant, D.M.
ACS division of Fuels, Preprint, 32, (4), 273-279, 1987. 8 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Solid state C-13 cross polarization (CP) magic angle spinning (MAS) NMR experiments have become very useful for the study of the structure of coal and other fossil fuels (1, 2). Solid-state NMR techniques have the advantage over liquid NMR methods because the whole solid coal can be analyzed in a nondestructive manner and no assumption need be made that the soluble portion represents the whole organic portion of the coal. From CP/MAS experiments twelve NMR structural parameters may be determined including the aromaticity, fa, that give information on the carbon skeletal backbone. The variation of the NMR structural parameters for coals of different rank from lignite to anthracite has been demonstrated (3). The coals in the Premium Coal Sample Bank (PCSB) have been included in the set of coals studied at the Advanced Combustion Engineering Research Center. The NMR data obtained on these coals will be used to provide chemical structural parameters for refinement of the devolatilization sub-model of the PCGC-2 combustion model.

This paper reports the twelve structural parameters for two of the PCSB coals, Pittsburgh #8 (HVB) and Blind Canyon (HVB). These data were derived from integrated intensities in a normal CP/MAS experiment along with a complete set of dipolar dephasing experiments. In addition, the proton spin lattice relaxation times for these coals have been determined. From relaxation experiments on the Pittsburgh #8 coal there is some evidence of heterogeneity between different sample vials of the same standard coal.

Non-Magic Angle Spinning NMR - An Approach to Determine the Chemical Shift Tensors in Char Particles

Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
ACS Div. of Chemistry, Preprints, 32, (4), 155-167, 1987. 13 pgs. Funded by Office of Basic Energy Science and US Department of Energy.

Carbon-13 NMR spectroscopy has been widely accepted as a major analytical technique for studying fossil fuels. Axelson and Davidson have reviewed the various solid state NMR spectroscopic techniques for coal studies. Cross polarization/magic angle spinning (CP/MAS) provides information on aromaticity while the dipolar dephasing technique (DD/MAS) provides additional definition of the structure of coal by using the C-13/H-1 dipolar coupling to separate the carbons into subclasses; i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aliphatic and aromatic regions of the spectrum. The combination of these two experimental procedures permits one to derive a carbon skeletal structure of coal samples.

MAS experiments produce narrow lines in solids but valuable structural information is lost; i.e., the chemical shift anisotropy (CSA) which is a manifestation of the three-dimensional shielding of the nucleus by the surrounding electrons. The CSA is a second-rank tensor having three principal elements s11, s22, and s33 characterized by unique resonance frequencies. These three tensor components taken together with the isotropic shielding value (the MAS value which is the average of the three tensor components) provide valuable data regarding the local electric environment. The tensor can be obtained from the C-13 NMR spectrum of a finely powdered sample and the tensor elements are extracted by analysis of the line shape. This technique provides not only tensor components but population values as well. However, the spectral analysis is complicated if more than one tensor is present and unique results are not always achievable. Evenso, Pines, et al. used the technique to analyze the static spectra of several coals. These workers succeeded in differentiating the contributions from aromatic and condensed aromatic carbons. Furthermore, they pointed out that there is little difference in the isotropic chemical shifts between these types of carbons and these shift differences cannot be resolved in a CP/MAS experiment. The DD/MAS experiment has been shown to differentiate between benzene-like (i.e., C-H) and non-protonated aromatic carbons (substituted plus inner, or bridgehead) but the resolution of substituted and inner carbons is not readily attainable with standard MAS experiments. The shielding anisotropies of the three general types of aromatic carbons are quite different and, in principal, should be resolvable in the "non-spinning" experiment.

Only for isolated carbons, or for simple compounds where breakpoints in the powder pattern are discernible, is it possible to obtain unique fits of the line shape. Tensor information can be extracted by other techniques such as analysis of the spinning side bands from slow spinning MAS experiments and variable angle sample spinning (VASS). In this study we have used the static and VASS methods to study pertinent model compounds and have used these data as a guide to analyze line shapes and extract CSA tensor values and population factors in coals. The experimental techniques are applicable to chars as well as coals. The data permit us to estimate the size of the polycondensed aromatic structure that is the main structural component in chars. The aromatic structure of coal chars should provide valuable information regarding their reactivity.

C-13 NMR Structural Determination of the Coals in the Premium Coal Sample Bank

Solum, M.S.; Pugmire, R.J. and Grant, D.M.
ACS division of Fuels, Preprint, 32, (4), 273-279, 1987. 8 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Solid state C-13 cross polarization (CP) magic angle spinning (MAS) NMR experiments have become very useful for the study of the structure of coal and other fossil fuels (1, 2). Solid-state NMR techniques have the advantage over liquid NMR methods because the whole solid coal can be analyzed in a nondestructive manner and no assumption need be made that the soluble portion represents the whole organic portion of the coal. From CP/MAS experiments twelve NMR structural parameters may be determined including the aromaticity, fa, that give information on the carbon skeletal backbone. The variation of the NMR structural parameters for coals of different rank from lignite to anthracite has been demonstrated (3). The coals in the Premium Coal Sample Bank (PCSB) have been included in the set of coals studied at the Advanced Combustion Engineering Research Center. The NMR data obtained on these coals will be used to provide chemical structural parameters for refinement of the devolatilization sub-model of the PCGC-2 combustion model.

This paper reports the twelve structural parameters for two of the PCSB coals, Pittsburgh #8 (HVB) and Blind Canyon (HVB). These data were derived from integrated intensities in a normal CP/MAS experiment along with a complete set of dipolar dephasing experiments. In addition, the proton spin lattice relaxation times for these coals have been determined. From relaxation experiments on the Pittsburgh #8 coal there is some evidence of heterogeneity between different sample vials of the same standard coal.

Non-Magic Angle Spinning NMR - An Approach to Determine the Chemical Shift Tensors in Char Particles

Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
ACS Div. of Chemistry, Preprints, 32, (4), 155-167, 1987. 13 pgs. Funded by Office of Basic Energy Science and US Department of Energy.

Carbon-13 NMR spectroscopy has been widely accepted as a major analytical technique for studying fossil fuels. Axelson and Davidson have reviewed the various solid state NMR spectroscopic techniques for coal studies. Cross polarization/magic angle spinning (CP/MAS) provides information on aromaticity while the dipolar dephasing technique (DD/MAS) provides additional definition of the structure of coal by using the C-13/H-1 dipolar coupling to separate the carbons into subclasses; i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aliphatic and aromatic regions of the spectrum. The combination of these two experimental procedures permits one to derive a carbon skeletal structure of coal samples.

MAS experiments produce narrow lines in solids but valuable structural information is lost; i.e., the chemical shift anisotropy (CSA) which is a manifestation of the three-dimensional shielding of the nucleus by the surrounding electrons. The CSA is a second-rank tensor having three principal elements s11, s22, and s33 characterized by unique resonance frequencies. These three tensor components taken together with the isotropic shielding value (the MAS value which is the average of the three tensor components) provide valuable data regarding the local electric environment. The tensor can be obtained from the C-13 NMR spectrum of a finely powdered sample and the tensor elements are extracted by analysis of the line shape. This technique provides not only tensor components but population values as well. However, the spectral analysis is complicated if more than one tensor is present and unique results are not always achievable. Evenso, Pines, et al. used the technique to analyze the static spectra of several coals. These workers succeeded in differentiating the contributions from aromatic and condensed aromatic carbons. Furthermore, they pointed out that there is little difference in the isotropic chemical shifts between these types of carbons and these shift differences cannot be resolved in a CP/MAS experiment. The DD/MAS experiment has been shown to differentiate between benzene-like (i.e., C-H) and non-protonated aromatic carbons (substituted plus inner, or bridgehead) but the resolution of substituted and inner carbons is not readily attainable with standard MAS experiments. The shielding anisotropies of the three general types of aromatic carbons are quite different and, in principal, should be resolvable in the "non-spinning" experiment.

Only for isolated carbons, or for simple compounds where breakpoints in the powder pattern are discernible, is it possible to obtain unique fits of the line shape. Tensor information can be extracted by other techniques such as analysis of the spinning side bands from slow spinning MAS experiments and variable angle sample spinning (VASS). In this study we have used the static and VASS methods to study pertinent model compounds and have used these data as a guide to analyze line shapes and extract CSA tensor values and population factors in coals. The experimental techniques are applicable to chars as well as coals. The data permit us to estimate the size of the polycondensed aromatic structure that is the main structural component in chars. The aromatic structure of coal chars should provide valuable information regarding their reactivity.

A Chemical Model of Coal Devolatilization Using Percolation Lattice Statistics

Grant, D.M.; Pugmire, R.J.; Fletcher, T.H. and Kerstein, A.R.
ACS Div. of Fuel Chemistry, 33, (2), 322-332, 1988. 10 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

We have developed a model for coal devolatilization that incorporates the diversity of coal structure in such a way that the analytical data obtained from solid state NMR provides the initial input data. Using experimentally determined kinetic rate parameters, it is possible to fit the gas, tar and char production of a lignite and high volatile bituminous coal. We have employed percolation theory to provide analytical expressions for the lattice statistics required in devolatilization modeling. The percolation theory allows one to avoid the more time-consuming Monte Carlo technique with no loss of generality or important statistical features. Percolation theory analytically describes the size distribution of finite clusters of sites joined by intact bridges but isolated from all remaining sites by broken bridges. The theory specifies a critical bridge population, depending only on the site coordination number, above which infinite arrays will coexist with clusters of finite size. It is a simple matter to adapt the structural features of percolation theory to both the tar and gas obtained in coal pyrolysis. The infinite arrays of percolation theory are interpreted as the macroscopic lattice of unreacted coal and/or char while the relatively small tar molecules may be identified with the fine clusters of percolation theory. The details of the model will be discussed together with the results obtained in modeling devolatilization behavior of coals of various ranks.

The Use of Variable Angle Sampling Spinning C-13 NMR Spectroscopy to Assess Aromatic Cluster Size in Coals, Coal Chars, and Carbonaceous Materials

Pugmire, R.J.; Sethi, N.K.; Solum, M.S.; Facelli, J.C. and Grant, D.M.
Proc. Carbon Conference, 1988. 3 pgs. Funded by US Department of Energy and Basic Energy Sciences.

Magic angle spinning experiments produce narrow lines in solids but valuable chemical shift anisotropy (CSA) information is lost as the chemical shift tensors are averaged to their isotropic values. CSA is a second rank tensor and is characterized by three unique resonance frequencies s11, s22 and s33 called the principal values of the tensor elements [1]. Knowledge of all three frequencies with the isotropic value is a source of information that is indispensable for molecular characterization. Measurement of the tensor elements is carried out by recording the C-13 NMR spectrum of a finely powdered sample to obtain the solid-state line shape which can then be analyzed to determine tensor elements of chemically and structurally different types of carbons. We have analyzed both static and variable angle sample spinning line shapes [2, 3] to obtain the CSA tensor values and population factors for the aromatic carbons of coals, macerals and carbonaceous materials from which aromatic cluster size is estimated.