Orendt, AM
1997
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.
1996
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.
1994
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.
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.
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 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.
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.
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 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.
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.
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.
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.
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).
1989
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.
1988
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.
Orendt, AM
1997
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.
1996
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.
1994
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.
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.
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 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.
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.
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 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.
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.
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.
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.
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).
1989
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.
1988
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.