Facelli, JC
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
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.
1994
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.
1993
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.
1992
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.
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
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.
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.
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.
1990
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.
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.
1989
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.
1988
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.
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.
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.
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.
Facelli, JC
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
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.
1994
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.
1993
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.
1992
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.
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
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.
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.
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.
1990
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.
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.
1989
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.
1988
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.
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.
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.
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.