Sethi, NK
1993
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.
1992
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.
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.
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.
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.
NMR Spectra from Powdered Solids Spinning at Any Angle and Speed: Simulations and Experiments
Sethi, N.K.; Alderman, D.W. and Grant, D.M.
Molecular Physics, 71 (2), 217-238, 1990. Funded by US Department of Energy.
A new method is described for calculating NMR spectra of powdered solids spinning at any speed and at any angle relative to the magnetic field. The algorithm provides the intensities and shapes of the center band and all sidebands through a computation involving the solution of homogeneous linear equations. The method is also applicable to cases where two or more inter-actions simultaneously affect the spectral response. A simple extension of the algorithm treats cases where an analytical expression for the resonance frequency is not available. The method is demonstrated with simulations and experimental examples of anisotropic chemical-shift spectra obtained by spinning samples at various low speeds and angles. The technique is also applied to a spin-1/2 nucleus with both an anisotropic chemical shift and a dipolar coupling either to a heteronuclear spin-1/2 nucleus or to a spin-1 nucleus experiencing a strong quadrupole interaction.
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
Polyethylene Crystallinity From Static, Solid-State NMR Spectra
Hughes, C.D.; Sethi, N.K.; Baltisberger, J.H. and Grant, D.M.
Macromolecules, 22, 2551, 1989. Funded by US Department of Energy.
A new method for determining the crystallinity of linear polyethylene (LPE) utilizing static, solid state C-13 Nuclear Magnetic Resonance (NMR) spectroscopy and computer lineshape simulation is described. Good agreement between the crystallinity measured by this method and a standard method was found. The computer simulation of the experimental spectrum also reflects the degree of gauche and trans chain conformations in the amorphous region of the sample.
1988-1987
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.
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.
C-13 Chemical Shielding Anisotropy Studied by Variable-Angle Sample Spinning
Sethi, N.K.; Grant, D.M. and Pugmire, R.J.
Journal of Magnetic Resonance, 71, 476-479, 1987. 4 pgs. Funded by US Department of Energy.
Spinning the sample at angles other than the magic angle is shown to have interesting applications for obtaining chemical shielding anisotropies for systems where severe overlapping makes the analysis of static powder pattern either cumbersome or impossible. Results for 1,3,5-trimethoxybenzene and p-dimethoxybenzene are in excellent agreement with those obtained via single-crystal studies.
Application of High Resolution Solid State NMR to Analysis of the Chemical Shift Anisotropy Tensors in Coals
Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
1987 International Conference of Coal Science, 1987, 41-44, Science, Holland, 1987. 4 pgs. Funded by US Department of Energy, Basic Energy Sciences, and ACERC (National Science Foundation and Associates and Affiliates).
Carbon-13 NMR spectroscopy has become an important tool in the structural determination of coal. The major techniques used are cross polarization magic angle spinning (CP/MAS) and dipolar dephasing (DD/MAS) experiments. The CP/MAS technique has been extensively used on both whole coals and macerals by this laboratory as a direct method for determining the fraction of aromatic carbons, fa, in these samples. The dipolar dephasing experiment (DD/MAS) provides additional definition of the structure of coal by using the C-13-H-1 dipolar coupling to separate the carbons into sub-classes, i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aromatic and aliphatic regions of the spectrum. The application of the technique to coals has been thoroughly described.
MAS experiments produce narrow lines in solids but, in doing so, valuable chemical shift anisotropy (CSA) information is lost as the chemical shift tensors are averaged to their isotropic values. CSA is a second rank tensor and is characterized by three unique resonance frequencies s11, s22 and s33 called the principal values of the tensor elements . Knowledge of all three frequencies in addition to the isotropic value (average of tensor elements) in a source of information which is indispensable for molecular characterization and independent peak assignments. Measurement of the tensor elements is carried out by recording the C-13 NMR spectrum of a finely powdered sample to obtain the solid state line shape which can then be analyzed to determine tensor elements of chemically and structurally different types of carbons. Other noteworthy techniques for extracting such data are analysis of spinning sidebands from slow magic angle spinning experiments and non-magic angle sample spinning referred to in this paper as variable angle sample spinning (VASS). In the present study, we have analyzed both static and variable angle sample spinning line shapes to obtain the CSA tensor values and population factors for the aromatic carbons of coals.
Non-Magic Angle Spinning NMR - An Approach to Determine the Chemical Shift Tensors in Char Particles
Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
ACS Div. of Chemistry, Preprints, 32, (4), 155-167, 1987. 13 pgs. Funded by Office of Basic Energy Science and US Department of Energy.
Carbon-13 NMR spectroscopy has been widely accepted as a major analytical technique for studying fossil fuels. Axelson and Davidson have reviewed the various solid state NMR spectroscopic techniques for coal studies. Cross polarization/magic angle spinning (CP/MAS) provides information on aromaticity while the dipolar dephasing technique (DD/MAS) provides additional definition of the structure of coal by using the C-13/H-1 dipolar coupling to separate the carbons into subclasses; i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aliphatic and aromatic regions of the spectrum. The combination of these two experimental procedures permits one to derive a carbon skeletal structure of coal samples.
MAS experiments produce narrow lines in solids but valuable structural information is lost; i.e., the chemical shift anisotropy (CSA) which is a manifestation of the three-dimensional shielding of the nucleus by the surrounding electrons. The CSA is a second-rank tensor having three principal elements s11, s22, and s33 characterized by unique resonance frequencies. These three tensor components taken together with the isotropic shielding value (the MAS value which is the average of the three tensor components) provide valuable data regarding the local electric environment. The tensor can be obtained from the C-13 NMR spectrum of a finely powdered sample and the tensor elements are extracted by analysis of the line shape. This technique provides not only tensor components but population values as well. However, the spectral analysis is complicated if more than one tensor is present and unique results are not always achievable. Evenso, Pines, et al. used the technique to analyze the static spectra of several coals. These workers succeeded in differentiating the contributions from aromatic and condensed aromatic carbons. Furthermore, they pointed out that there is little difference in the isotropic chemical shifts between these types of carbons and these shift differences cannot be resolved in a CP/MAS experiment. The DD/MAS experiment has been shown to differentiate between benzene-like (i.e., C-H) and non-protonated aromatic carbons (substituted plus inner, or bridgehead) but the resolution of substituted and inner carbons is not readily attainable with standard MAS experiments. The shielding anisotropies of the three general types of aromatic carbons are quite different and, in principal, should be resolvable in the "non-spinning" experiment.
Only for isolated carbons, or for simple compounds where breakpoints in the powder pattern are discernible, is it possible to obtain unique fits of the line shape. Tensor information can be extracted by other techniques such as analysis of the spinning side bands from slow spinning MAS experiments and variable angle sample spinning (VASS). In this study we have used the static and VASS methods to study pertinent model compounds and have used these data as a guide to analyze line shapes and extract CSA tensor values and population factors in coals. The experimental techniques are applicable to chars as well as coals. The data permit us to estimate the size of the polycondensed aromatic structure that is the main structural component in chars. The aromatic structure of coal chars should provide valuable information regarding their reactivity.
Sethi, NK
1993
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.
1992
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.
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.
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.
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.
NMR Spectra from Powdered Solids Spinning at Any Angle and Speed: Simulations and Experiments
Sethi, N.K.; Alderman, D.W. and Grant, D.M.
Molecular Physics, 71 (2), 217-238, 1990. Funded by US Department of Energy.
A new method is described for calculating NMR spectra of powdered solids spinning at any speed and at any angle relative to the magnetic field. The algorithm provides the intensities and shapes of the center band and all sidebands through a computation involving the solution of homogeneous linear equations. The method is also applicable to cases where two or more inter-actions simultaneously affect the spectral response. A simple extension of the algorithm treats cases where an analytical expression for the resonance frequency is not available. The method is demonstrated with simulations and experimental examples of anisotropic chemical-shift spectra obtained by spinning samples at various low speeds and angles. The technique is also applied to a spin-1/2 nucleus with both an anisotropic chemical shift and a dipolar coupling either to a heteronuclear spin-1/2 nucleus or to a spin-1 nucleus experiencing a strong quadrupole interaction.
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
Polyethylene Crystallinity From Static, Solid-State NMR Spectra
Hughes, C.D.; Sethi, N.K.; Baltisberger, J.H. and Grant, D.M.
Macromolecules, 22, 2551, 1989. Funded by US Department of Energy.
A new method for determining the crystallinity of linear polyethylene (LPE) utilizing static, solid state C-13 Nuclear Magnetic Resonance (NMR) spectroscopy and computer lineshape simulation is described. Good agreement between the crystallinity measured by this method and a standard method was found. The computer simulation of the experimental spectrum also reflects the degree of gauche and trans chain conformations in the amorphous region of the sample.
1988-1987
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.
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.
C-13 Chemical Shielding Anisotropy Studied by Variable-Angle Sample Spinning
Sethi, N.K.; Grant, D.M. and Pugmire, R.J.
Journal of Magnetic Resonance, 71, 476-479, 1987. 4 pgs. Funded by US Department of Energy.
Spinning the sample at angles other than the magic angle is shown to have interesting applications for obtaining chemical shielding anisotropies for systems where severe overlapping makes the analysis of static powder pattern either cumbersome or impossible. Results for 1,3,5-trimethoxybenzene and p-dimethoxybenzene are in excellent agreement with those obtained via single-crystal studies.
Application of High Resolution Solid State NMR to Analysis of the Chemical Shift Anisotropy Tensors in Coals
Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
1987 International Conference of Coal Science, 1987, 41-44, Science, Holland, 1987. 4 pgs. Funded by US Department of Energy, Basic Energy Sciences, and ACERC (National Science Foundation and Associates and Affiliates).
Carbon-13 NMR spectroscopy has become an important tool in the structural determination of coal. The major techniques used are cross polarization magic angle spinning (CP/MAS) and dipolar dephasing (DD/MAS) experiments. The CP/MAS technique has been extensively used on both whole coals and macerals by this laboratory as a direct method for determining the fraction of aromatic carbons, fa, in these samples. The dipolar dephasing experiment (DD/MAS) provides additional definition of the structure of coal by using the C-13-H-1 dipolar coupling to separate the carbons into sub-classes, i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aromatic and aliphatic regions of the spectrum. The application of the technique to coals has been thoroughly described.
MAS experiments produce narrow lines in solids but, in doing so, valuable chemical shift anisotropy (CSA) information is lost as the chemical shift tensors are averaged to their isotropic values. CSA is a second rank tensor and is characterized by three unique resonance frequencies s11, s22 and s33 called the principal values of the tensor elements . Knowledge of all three frequencies in addition to the isotropic value (average of tensor elements) in a source of information which is indispensable for molecular characterization and independent peak assignments. Measurement of the tensor elements is carried out by recording the C-13 NMR spectrum of a finely powdered sample to obtain the solid state line shape which can then be analyzed to determine tensor elements of chemically and structurally different types of carbons. Other noteworthy techniques for extracting such data are analysis of spinning sidebands from slow magic angle spinning experiments and non-magic angle sample spinning referred to in this paper as variable angle sample spinning (VASS). In the present study, we have analyzed both static and variable angle sample spinning line shapes to obtain the CSA tensor values and population factors for the aromatic carbons of coals.
Non-Magic Angle Spinning NMR - An Approach to Determine the Chemical Shift Tensors in Char Particles
Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
ACS Div. of Chemistry, Preprints, 32, (4), 155-167, 1987. 13 pgs. Funded by Office of Basic Energy Science and US Department of Energy.
Carbon-13 NMR spectroscopy has been widely accepted as a major analytical technique for studying fossil fuels. Axelson and Davidson have reviewed the various solid state NMR spectroscopic techniques for coal studies. Cross polarization/magic angle spinning (CP/MAS) provides information on aromaticity while the dipolar dephasing technique (DD/MAS) provides additional definition of the structure of coal by using the C-13/H-1 dipolar coupling to separate the carbons into subclasses; i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aliphatic and aromatic regions of the spectrum. The combination of these two experimental procedures permits one to derive a carbon skeletal structure of coal samples.
MAS experiments produce narrow lines in solids but valuable structural information is lost; i.e., the chemical shift anisotropy (CSA) which is a manifestation of the three-dimensional shielding of the nucleus by the surrounding electrons. The CSA is a second-rank tensor having three principal elements s11, s22, and s33 characterized by unique resonance frequencies. These three tensor components taken together with the isotropic shielding value (the MAS value which is the average of the three tensor components) provide valuable data regarding the local electric environment. The tensor can be obtained from the C-13 NMR spectrum of a finely powdered sample and the tensor elements are extracted by analysis of the line shape. This technique provides not only tensor components but population values as well. However, the spectral analysis is complicated if more than one tensor is present and unique results are not always achievable. Evenso, Pines, et al. used the technique to analyze the static spectra of several coals. These workers succeeded in differentiating the contributions from aromatic and condensed aromatic carbons. Furthermore, they pointed out that there is little difference in the isotropic chemical shifts between these types of carbons and these shift differences cannot be resolved in a CP/MAS experiment. The DD/MAS experiment has been shown to differentiate between benzene-like (i.e., C-H) and non-protonated aromatic carbons (substituted plus inner, or bridgehead) but the resolution of substituted and inner carbons is not readily attainable with standard MAS experiments. The shielding anisotropies of the three general types of aromatic carbons are quite different and, in principal, should be resolvable in the "non-spinning" experiment.
Only for isolated carbons, or for simple compounds where breakpoints in the powder pattern are discernible, is it possible to obtain unique fits of the line shape. Tensor information can be extracted by other techniques such as analysis of the spinning side bands from slow spinning MAS experiments and variable angle sample spinning (VASS). In this study we have used the static and VASS methods to study pertinent model compounds and have used these data as a guide to analyze line shapes and extract CSA tensor values and population factors in coals. The experimental techniques are applicable to chars as well as coals. The data permit us to estimate the size of the polycondensed aromatic structure that is the main structural component in chars. The aromatic structure of coal chars should provide valuable information regarding their reactivity.