Alderman, DW
1997
A High-Resolution 3D Separated-Local-Field Experiment by Means of Magic-Angle Turning
Hu, J.Z.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 126:120-26(1997). Funded by US Department of Energy.
A 3D separated-local-field (SLF) experiment based on the 2D PHORMAT technique is described. In the 3D experiment, the conventional 2D SLF powder pattern for each chemically inequivalent carbon is separated according to their different isotropic chemical shifts. The dipolar coupling constant of a C-H pair, hence the bond distance, and the relative orientation of the chemical-shift tensor tot eh C-H vector can all be determined for the protonated carbons with a single measurement. As the sample turns at only about 30 Hz in a MAT experiment, the SLF patterns obtained approach those of a stationary sample, and an accuracy in the measurement similar to that obtained on a stationary sample is expected. The technique is demonstrated on 2,6-dimethoxynaphthalene, where the C-13-H-1 separated-local-field powder patterns for the six chemically inequivalent carbons are clearly identified and measured. The observed dipolar coupling for the mothoxy carbons is effectively reduced by the fast rotation of the group about its C3 symmetry axis. The average angle between the C-H bond direction and the C3 rotation axis in the OCH3 group is found to be about 66°.
1994
Magic Angle Turning Experiments for Measuring Chemical-Shift-Tensor Principal Values in Powdered Solids
Hu, J.Z.; Wang, W.; Liu, F.; Solum, M.S.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
J. Magnetic Resonance, 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.
The magic-angle-turning (MAT) technique introduced by Gan employs slow (ca. 30 Hz) rotation of a powdered sample at the magic angle, in concert with pulses synchronized at 1/3 of the rotor period, to obtain isotropic-shift information in one dimension of a 2D spectrum. The other dimension displays a slow-spinning-sideband powder pattern that, at the low rotor frequencies employed, resembles the stationary-sample powder pattern. The MAT method is very effective for measuring chemical-shift principal values in compounds where spectral overlap precludes the use of 1D methods. Previous MAT implementations are reviewed, and it is shown how a new phase correct MAT (PHORMAT) pulse sequence overcomes many of their limitations. This new pulse sequence produces a spinning-sideband-free isotropic-shift spectrum directly as a projection onto the evolution axis with no spectral shearing. Only two purging operations are employed, resulting in a higher signal-to-noise ratio. Pure absorption-absorption phased 2D spectra are produced. Flat 2D base planes result from an echo sequence which delays acquisition until after probe ring-down and receiver recovery. The technique used for synchronizing the pulses to 1/3 the rotor period without relying on absolute rotor-frequency stability is described. The PHORMAT spectrum of methyl a-D-glucopyranoside is presented. The data are analyzed with an emphasis on the quantitative accuracy of the experiment in measuring chemical shift tensor principal values and determining the relative number of spins of each type present. The FID data from the spectrometer acquisition are fitted with numerical simulations that employ a banded-matrix method for calculation spinning sideband amplitudes. The chemical shift principal values, measured in methyl a-D-glucopyranoside with the PHORMAT method, are compared with those from a single-crystal determination of the full chemical shift tensors. The two measurements differ by an rms average distance of only 0.57 ppm.
Measurement of C-13 Chemical Shift Tensor Principal Values with a Magical Angle Turning Experiment
Hu, J.Z.; Orendt, A.M.; Alderman, D.W.; Pugmire, R.J.; Ye, C. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 5:181, 1994. Funded by US Department of Energy and ACERC.
The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of C-13 chemical shift tensors in powdered solids. A large-volume MAT prove with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively, can enhance measurements of the principal values of chemical shift tensors in complex compounds. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the C-13 principal values in 2,3-dimethlnaphthalene and Pocohontas coal are reported at typical examples.
The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment
Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
ACS Preprints, Division of Fuel Chemistry, 39:8-112, 1994. Funded by Pittsburg Energy Technology Center, US Department of Energy and ACERC.
The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high-resolution spectra in complex solids such as coal. MAS narrows the chemical shift anisotropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values. In and effort to address this problem of spectral overlap, many 2D techniques have been developed to simultaneously obtain the dispersion by isotropic shift, such as produced by MAS, in one dimension and the tensorial information as a separate powder patters in the second dimension. A very successful technique is the slow spinning modification of the magic angle hopping experiment recently proposed by Gan, which we call the Magic Angle Turning (MAT) experiment. This experiment has a number of advantages over earlier 2D methods. The use of very slow spinning speeds (<50 Hz) favors the quantitative polarization of all carbons and allows the use of a large volume sample rotor resulting in a typical 2D spectrum acquisition requiring less than 24 hours. The mechanical device for slow spinning is very stable and high resolution in the isotropic chemical shift dimension can be easily obtained. The MAT experiment could be done on a suitably stable MAS probe. The only disadvantages of the original MAT experiment is that data acquisition starts right after the last pulse, causing distortion in the evolution dimension (the second dimension) even if a delay as short as 20 ms is used.
In this paper, a triple-echo MAT sequence, previously described, is employed which improves the 2D baseline. Two additional experiments, using short contact times and dipolar dephasing techniques, are also employed to further separate the powder patterns of protonated and nonprotonated carbons in complex compounds. Experimental results on representative model compounds as well as coals are presented in this paper.
1993
Improvements to the Magic Angle Hopping Experiment
Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 2:235-243, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.
Several improvements to the magic angle hopping experiment first introduced by Bax et al. (J. Magn. Reason., 52 (1983) 147 are presented. A dc servo motor driven sample hopping mechanism which requires less than 60 ms to accomplish a 120º sample rotation is described. Modifications to the data acquisition process, including starting the acquisition period immediately after the second hop and acquiring a hypercomplex data set, are also presented. Principal values of the C-13 chemical shielding tensor are measured for 1,2,3-trimethoxybenzene and 2,6-dimethoxynaphthalene.
An Isotropic Chemical Shift-Chemical Shift Anisotropy Magic-Angle Slow-Spinning 2-D NMR Experiment
Hu, J.Z.; Alderman, D.W.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 31: 472, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.
High-speed magic-angle spinning has become a ubiquitous method for obtaining high-resolution spectra in polycrystalline and amorphous solids. MAS reduces a chemical-shift-anisotropy powder pattern to a single line at the isotropic shift when the sample spinning speed is larger than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the three principal values of the tensor are even more valuable. These principal values are available in the powder pattern obtained from a stationary or slowly spinning sample provided the molecule has few enough unique nuclei that the spectrum can be interpreted. Unfortunately, overlap of several broad powder patterns often prevents the separation necessary for their individual identification and measurement.
Recognizing that an equivalent of the hopping experiment could be achieved without stopping the sample, Gan has recently demonstrated an elegant technique that employs pulses spaced at one-third of the rotor period to produce 2D spectra with an isotropic shift projection along the evolution dimension and an undistorted slow-spinning-sideband powder pattern in the acquisition dimension. The pulse sequence produces an isotropic shift dimension completely free of sidebands even in the slow-spinning regime. Because it requires only a slow continuous rotation of the sample, Gan's experiment is much easier to perform than the hopping experiment. However, in the process of projecting the magnetization onto the longitudinal axis twice during the rotor period, only one-fourth of the magnetization is retained and the sensitivity of the experiment is reduced accordingly. Described here is a technique which uses the same slow rotation of the sample about an axis at the magic angle, but instead applies five pi (180º) pulses to the magnetization Precessing in the transverse plane. The pulse sequence produces a result similar to that of Gan's experiment without the sacrifice of any magnetization to projections out of the transverse plane. This "5p" pulse method can be viewed simply as a constant-time version of Gan's experiment, with resultant advantages and disadvantages to be discussed.
Comparing, Modeling, and Assigning Chemical-Shift Tensors in the Cartestian, Irreducible Spherical, and Icosahedral Representations
Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Journal of Magnetic Resonance, 101:188-197, 1993. Funded by US Department of Energy, Division of Chemical Sciences and Office of Basic Energy Sciences.
A measure of the difference between two chemical-shift tensors is developed by defining the scalar distance between them. Chemical-shift tensors are treated as functions whose domain is the surface of a sphere and the mathematical definition of the quadratic distance between two functions is invoked. Expressions for the distance between two chemical-shift tensors are developed in the Cartesian and irreducible spherical representations and in a new icosahedral representation. A representation wherein the chemical-shift tensor is specified by the shifts when the magnetic field is along six directions defined by the vertices of an icosahedron is developed and its properties are discussed. The expression for the distance between two tensors is found to be particularly attractive and useful in this icosahedral representation. The distance between tensors computed in the icosahedral representation is useful in fitting linear models to tensor data. It is shown how such fitting can contribute to the assignment of tensors obtained from single-crystal studies. A quantitative figure of merit useful for comparing multiple assignment possibilities is developed. The results derived are applicable to any physical phenomenon described by real zero-rank and second-rank tensors.
Chemical-Shift-Chemical-Shift Correlation Spectroscopy in Powdered Solids
Hughes, C.D.; Sherwood, M.H.; Alderman, D.W. and Grant, D.M.
Journal of Magnetic Resonance, 102:58-72, 1993. Funded by US Department of Energy.
Sample reorientation during the mixing time of a two-dimensional NMR exchange experiment allows the measurement of chemical-shift tensors in powdered samples whose one-dimensional powder patterns are too complex to reliably analyze. This technique has the advantages that the chemical-shift anisotropy has an unscaled representation, the artifacts caused by cross polarization, spin relaxation, and partial sample orientation can be accounted for, the experiment is relatively easy to perform, and no spinning sidebands interfere. The method is useful for sorting out the principal values of overlapping chemical-shift tensors.
The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment
Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by US Department of Energy and Pittsburgh Energy Technology Center.
The chemical shift of a C-13 spin in a solid sample varies with the change of the relative orientation of the nuclei (or molecule) to the external magnetic field. This orientational dependence produces the well-known chemical shift anisotropy (CSA). The tensor values of the CSA provides a wealth of information about subtle differences in the electronic environment of the nuclei, such as the type of bond, the effects of electron devolatilization and the bond conformation as well as the dynamics of the nuclei. The principal values of the CSA can be obtained in a straightforward way when the molecule has only a few, e.g., 2-3 unique nuclei. In most substances, unfortunately, the overlap of the several broad powder patterns prevents the spectral separation necessary for individual carbon resolution and identification.
The Magic Angle Turning Experiment. A New NMR Tool for Studying Coal Structure
Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
10th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 1993. Funded by US Department of Energy and ACERC.
The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high resolution spectra in complex solids such as coal. MAS narrows the chemical shift anistropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anistropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values.
1992
Chemical Shift-Chemical Shift Correlation Spectroscopy in Powdered Solids
Hughes, C.D.; Sherwood, M.H.; Alderman, D.W. and Grant, D.M.
Fifth Coal Chemistry Conference and Workshop, Rockford, IL, June 1992. Funded by US Department of Energy.
Sample reorientation during the mixing time of a two-dimensional NMR exchange experiment allows the measurements of chemical shift tensors in powdered samples whose one-dimensional powder patterns are too complex to reliably analyze. This technique has the advantages that the chemical shift anisotropy has an unscaled representation, the artifacts caused by cross polarization, spin relaxation and partial sample orientation can be accounted for, the experiment is relatively easy to perform, and no spinning sidebands interfere. The method is useful for sorting out the principal values of overlapping chemical shift tensors.
1991
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.
1990
Two-Dimensional Chemical-Shift Tensor Correlation Spectroscopy. Analysis of Sensitivity and Optimal Measurement Directions
Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Journal of Magnetic Resonance, 86, 60-69, 1990. Funded by US Department of Energy.
Chemical-shift tensors can be determined from a single crystal placed in six or more orientations in a magnetic field. The sensitivity of this determination as a function of the selected crystal orientations is considered with a statistical figure of merit. A variety of configurations are examined, and it is found that the set of six orientations based on the vertices of an icosahedron optimizes the figure of merit and results in the most sensitive determination of the tensor. The relationship of these optimal orientations to those used previously in two-dimensional chemical-shift tensor correlation spectroscopy is discussed. It is shown that the high symmetry of the icosahedron simplifies the design on which the construction of a new sample orienting mechanism may be based.
1989
Two-Dimensional Chemical Shift Tensor Correlation Spectroscopy. Analysis of Sensitivity and Optimal Measurement Directions
Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Accepted for publication in J. Mag. Res., 1989. Funded by US Department of Energy.
Chemical shift tensors can be determined from a single crystal placed in six or more orientations in a magnetic field. The sensitivity of this determination as a function of the selected crystal orientations is considered with a statistical figure of merit. A variety of configurations are examined, and it is found that the set of six orientations based on the vertices of an icosahedron optimizes the figure of merit and results in the most sensitive determination of the tensor. The relationship of these optimal orientations to those used previously in two-dimensional chemical shift tensor correlation spectroscopy is discussed. It is shown that the high symmetry of the icosahedron simplifies the design on which the construction of a new sample orienting mechanism may be based.
1988
Efficient Stator/Rotor Assembly for Magic-Angle Spinning NMR
Jiang, Y.J.; Woolfenden, W.R.; Alderman, D.W.; Mayne, C.L.; Pugmire, R.J. and Grant, D.M.
Rev. SCI Instrum, 58, (5), 755-758, 1987. 4 pgs. Funded by US Department of Energy and Office of Basic Energy Sciences.
A newly designed stator assembly for cylindrical spinners used in magic-angle spinning nuclear-magnetic resonance experiments is described. Separate driving and bearing gas chambers allow variable and stable spinning speeds, and this design permits easy starting and stopping of the rotor. Isolation of the chambers is achieved with the application of pressure screws rather than O-rings or glue lines to avoid leakage at high gas pressures. The overall dimensions are optimal to facilitate easy assembly. Some significant modifications have been made to an earlier spinner design. These improvements give better efficiency and concentricity of the spinner. Applications are illustrated with carbon-13 cross polarization/magic angle spinning (CP/MAS) spectra carried out at different rotor spinning rates.
Carbon-13 Chemical Shift Tensors in Single-Crystal Methoxybenzenes
Carter, C.M.; Facelli, J.C.; Alderman, D.W.; Grant, D.M.; Dalley, N.K. and Wilson, B.E.
J. Chem. Soc. Faraday Trans. 1, 1988. Funded by US Department of Energy.
Using a two-dimensional NMR orientation correlation technique carbon-13 chemical-shift tensors have been measured in single crystals of 1.4-dimethoxybenzene. 1,3,5-trimethoxybenzene and 1,2,3-trimethosybenzene. The two-dimensional technique greatly extends the single-crystal method to materials with a much larger number of different carbon-13 resonance lines. The manner of dealing with chemically identical, but magnetically non-equivalent carbons in the unit cell is discussed. The X-ray structure of 1, 2, 3-trimethoxybenzene is reported for the first time, and a redetermination of the X-ray structure for 1.4-dimethoxybenzene is given. The principal values of all carbon-13 chemical-shift tensors in the three molecules and the orientation of their principal axes have been obtained. Using multiple regressional analysis the principal values of the tensors have been discussed in terms of additive substituent effects. AB initio calculations of the shielding tensors in anisole (methoxybenzene) and benzene were used to calculate substituent effects that agree closely with the parameters obtained from the regressional analysis.
Double Resonance Interferometry, Relaxation Modulated Coherence Transfer
Newmark, R.D.; Alderman, D.W. and Grant, D.M.
Submitted to Phys. Rev., 1988. Funded by US Department of Energy.
The double resonance interferometry experiment of Stoll, Vega and Vaughan is examined with numerical simulations employing the complete Redfield matrix. It is shown that their observed doublet asymmetry is a consequence of random field relaxation mechanisms. The relaxation pathways in the system are examined in detail to explain the coherence transfer that takes place during the SVV experiment. The simulations must include relaxation during the selective carbon-13 pulses in order to agree with the experimental results thereby indicating the need for care in using impulse approximations for selective pulses in some double resonance experiments.
Time-Domain Fitting and Director Reorientation of H-1-NMR Spectra of Molecules Dissolved in Liquid-Crystal Solvents
Heeschen, W.A.; Alderman, D.W. and Grant, D.M.
Submitted to J. Phys. Chem, 1988. Funded by US Department of Energy.
A time-domain fitting routine is described and applied to the analysis of a H-1-NMR spectrum of a solute in a liquid-crystalline solvent. Principles of the time-domain fitting technique are described in terms of data selection, baseline compensation and computational advantage over other fitting methods. Also, the use of fitting coefficients based on structural variation is described and found to provide rapid, reliable spectrum fits with physically reasonable structures. Solute ordering information is determined through interpretation of high-resolution spectra using director reorientation by the variable-angle sample spinning technique to provide a starting point for the fitting. Spectra from norbornadiene dissolved in the liquid crystal p-pentylphenyl 2-chloro-4-(p-pentylbenzoyloxy) benzoate are used to demonstrate the method. The resultant fit of the norbornadiene structure is compared to results obtained previously by other workers.
Alderman, DW
1997
A High-Resolution 3D Separated-Local-Field Experiment by Means of Magic-Angle Turning
Hu, J.Z.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 126:120-26(1997). Funded by US Department of Energy.
A 3D separated-local-field (SLF) experiment based on the 2D PHORMAT technique is described. In the 3D experiment, the conventional 2D SLF powder pattern for each chemically inequivalent carbon is separated according to their different isotropic chemical shifts. The dipolar coupling constant of a C-H pair, hence the bond distance, and the relative orientation of the chemical-shift tensor tot eh C-H vector can all be determined for the protonated carbons with a single measurement. As the sample turns at only about 30 Hz in a MAT experiment, the SLF patterns obtained approach those of a stationary sample, and an accuracy in the measurement similar to that obtained on a stationary sample is expected. The technique is demonstrated on 2,6-dimethoxynaphthalene, where the C-13-H-1 separated-local-field powder patterns for the six chemically inequivalent carbons are clearly identified and measured. The observed dipolar coupling for the mothoxy carbons is effectively reduced by the fast rotation of the group about its C3 symmetry axis. The average angle between the C-H bond direction and the C3 rotation axis in the OCH3 group is found to be about 66°.
1994
Magic Angle Turning Experiments for Measuring Chemical-Shift-Tensor Principal Values in Powdered Solids
Hu, J.Z.; Wang, W.; Liu, F.; Solum, M.S.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
J. Magnetic Resonance, 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.
The magic-angle-turning (MAT) technique introduced by Gan employs slow (ca. 30 Hz) rotation of a powdered sample at the magic angle, in concert with pulses synchronized at 1/3 of the rotor period, to obtain isotropic-shift information in one dimension of a 2D spectrum. The other dimension displays a slow-spinning-sideband powder pattern that, at the low rotor frequencies employed, resembles the stationary-sample powder pattern. The MAT method is very effective for measuring chemical-shift principal values in compounds where spectral overlap precludes the use of 1D methods. Previous MAT implementations are reviewed, and it is shown how a new phase correct MAT (PHORMAT) pulse sequence overcomes many of their limitations. This new pulse sequence produces a spinning-sideband-free isotropic-shift spectrum directly as a projection onto the evolution axis with no spectral shearing. Only two purging operations are employed, resulting in a higher signal-to-noise ratio. Pure absorption-absorption phased 2D spectra are produced. Flat 2D base planes result from an echo sequence which delays acquisition until after probe ring-down and receiver recovery. The technique used for synchronizing the pulses to 1/3 the rotor period without relying on absolute rotor-frequency stability is described. The PHORMAT spectrum of methyl a-D-glucopyranoside is presented. The data are analyzed with an emphasis on the quantitative accuracy of the experiment in measuring chemical shift tensor principal values and determining the relative number of spins of each type present. The FID data from the spectrometer acquisition are fitted with numerical simulations that employ a banded-matrix method for calculation spinning sideband amplitudes. The chemical shift principal values, measured in methyl a-D-glucopyranoside with the PHORMAT method, are compared with those from a single-crystal determination of the full chemical shift tensors. The two measurements differ by an rms average distance of only 0.57 ppm.
Measurement of C-13 Chemical Shift Tensor Principal Values with a Magical Angle Turning Experiment
Hu, J.Z.; Orendt, A.M.; Alderman, D.W.; Pugmire, R.J.; Ye, C. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 5:181, 1994. Funded by US Department of Energy and ACERC.
The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of C-13 chemical shift tensors in powdered solids. A large-volume MAT prove with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively, can enhance measurements of the principal values of chemical shift tensors in complex compounds. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the C-13 principal values in 2,3-dimethlnaphthalene and Pocohontas coal are reported at typical examples.
The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment
Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
ACS Preprints, Division of Fuel Chemistry, 39:8-112, 1994. Funded by Pittsburg Energy Technology Center, US Department of Energy and ACERC.
The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high-resolution spectra in complex solids such as coal. MAS narrows the chemical shift anisotropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values. In and effort to address this problem of spectral overlap, many 2D techniques have been developed to simultaneously obtain the dispersion by isotropic shift, such as produced by MAS, in one dimension and the tensorial information as a separate powder patters in the second dimension. A very successful technique is the slow spinning modification of the magic angle hopping experiment recently proposed by Gan, which we call the Magic Angle Turning (MAT) experiment. This experiment has a number of advantages over earlier 2D methods. The use of very slow spinning speeds (<50 Hz) favors the quantitative polarization of all carbons and allows the use of a large volume sample rotor resulting in a typical 2D spectrum acquisition requiring less than 24 hours. The mechanical device for slow spinning is very stable and high resolution in the isotropic chemical shift dimension can be easily obtained. The MAT experiment could be done on a suitably stable MAS probe. The only disadvantages of the original MAT experiment is that data acquisition starts right after the last pulse, causing distortion in the evolution dimension (the second dimension) even if a delay as short as 20 ms is used.
In this paper, a triple-echo MAT sequence, previously described, is employed which improves the 2D baseline. Two additional experiments, using short contact times and dipolar dephasing techniques, are also employed to further separate the powder patterns of protonated and nonprotonated carbons in complex compounds. Experimental results on representative model compounds as well as coals are presented in this paper.
1993
Improvements to the Magic Angle Hopping Experiment
Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 2:235-243, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.
Several improvements to the magic angle hopping experiment first introduced by Bax et al. (J. Magn. Reason., 52 (1983) 147 are presented. A dc servo motor driven sample hopping mechanism which requires less than 60 ms to accomplish a 120º sample rotation is described. Modifications to the data acquisition process, including starting the acquisition period immediately after the second hop and acquiring a hypercomplex data set, are also presented. Principal values of the C-13 chemical shielding tensor are measured for 1,2,3-trimethoxybenzene and 2,6-dimethoxynaphthalene.
An Isotropic Chemical Shift-Chemical Shift Anisotropy Magic-Angle Slow-Spinning 2-D NMR Experiment
Hu, J.Z.; Alderman, D.W.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 31: 472, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.
High-speed magic-angle spinning has become a ubiquitous method for obtaining high-resolution spectra in polycrystalline and amorphous solids. MAS reduces a chemical-shift-anisotropy powder pattern to a single line at the isotropic shift when the sample spinning speed is larger than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the three principal values of the tensor are even more valuable. These principal values are available in the powder pattern obtained from a stationary or slowly spinning sample provided the molecule has few enough unique nuclei that the spectrum can be interpreted. Unfortunately, overlap of several broad powder patterns often prevents the separation necessary for their individual identification and measurement.
Recognizing that an equivalent of the hopping experiment could be achieved without stopping the sample, Gan has recently demonstrated an elegant technique that employs pulses spaced at one-third of the rotor period to produce 2D spectra with an isotropic shift projection along the evolution dimension and an undistorted slow-spinning-sideband powder pattern in the acquisition dimension. The pulse sequence produces an isotropic shift dimension completely free of sidebands even in the slow-spinning regime. Because it requires only a slow continuous rotation of the sample, Gan's experiment is much easier to perform than the hopping experiment. However, in the process of projecting the magnetization onto the longitudinal axis twice during the rotor period, only one-fourth of the magnetization is retained and the sensitivity of the experiment is reduced accordingly. Described here is a technique which uses the same slow rotation of the sample about an axis at the magic angle, but instead applies five pi (180º) pulses to the magnetization Precessing in the transverse plane. The pulse sequence produces a result similar to that of Gan's experiment without the sacrifice of any magnetization to projections out of the transverse plane. This "5p" pulse method can be viewed simply as a constant-time version of Gan's experiment, with resultant advantages and disadvantages to be discussed.
Comparing, Modeling, and Assigning Chemical-Shift Tensors in the Cartestian, Irreducible Spherical, and Icosahedral Representations
Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Journal of Magnetic Resonance, 101:188-197, 1993. Funded by US Department of Energy, Division of Chemical Sciences and Office of Basic Energy Sciences.
A measure of the difference between two chemical-shift tensors is developed by defining the scalar distance between them. Chemical-shift tensors are treated as functions whose domain is the surface of a sphere and the mathematical definition of the quadratic distance between two functions is invoked. Expressions for the distance between two chemical-shift tensors are developed in the Cartesian and irreducible spherical representations and in a new icosahedral representation. A representation wherein the chemical-shift tensor is specified by the shifts when the magnetic field is along six directions defined by the vertices of an icosahedron is developed and its properties are discussed. The expression for the distance between two tensors is found to be particularly attractive and useful in this icosahedral representation. The distance between tensors computed in the icosahedral representation is useful in fitting linear models to tensor data. It is shown how such fitting can contribute to the assignment of tensors obtained from single-crystal studies. A quantitative figure of merit useful for comparing multiple assignment possibilities is developed. The results derived are applicable to any physical phenomenon described by real zero-rank and second-rank tensors.
Chemical-Shift-Chemical-Shift Correlation Spectroscopy in Powdered Solids
Hughes, C.D.; Sherwood, M.H.; Alderman, D.W. and Grant, D.M.
Journal of Magnetic Resonance, 102:58-72, 1993. Funded by US Department of Energy.
Sample reorientation during the mixing time of a two-dimensional NMR exchange experiment allows the measurement of chemical-shift tensors in powdered samples whose one-dimensional powder patterns are too complex to reliably analyze. This technique has the advantages that the chemical-shift anisotropy has an unscaled representation, the artifacts caused by cross polarization, spin relaxation, and partial sample orientation can be accounted for, the experiment is relatively easy to perform, and no spinning sidebands interfere. The method is useful for sorting out the principal values of overlapping chemical-shift tensors.
The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment
Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by US Department of Energy and Pittsburgh Energy Technology Center.
The chemical shift of a C-13 spin in a solid sample varies with the change of the relative orientation of the nuclei (or molecule) to the external magnetic field. This orientational dependence produces the well-known chemical shift anisotropy (CSA). The tensor values of the CSA provides a wealth of information about subtle differences in the electronic environment of the nuclei, such as the type of bond, the effects of electron devolatilization and the bond conformation as well as the dynamics of the nuclei. The principal values of the CSA can be obtained in a straightforward way when the molecule has only a few, e.g., 2-3 unique nuclei. In most substances, unfortunately, the overlap of the several broad powder patterns prevents the spectral separation necessary for individual carbon resolution and identification.
The Magic Angle Turning Experiment. A New NMR Tool for Studying Coal Structure
Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
10th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 1993. Funded by US Department of Energy and ACERC.
The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high resolution spectra in complex solids such as coal. MAS narrows the chemical shift anistropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anistropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values.
1992
Chemical Shift-Chemical Shift Correlation Spectroscopy in Powdered Solids
Hughes, C.D.; Sherwood, M.H.; Alderman, D.W. and Grant, D.M.
Fifth Coal Chemistry Conference and Workshop, Rockford, IL, June 1992. Funded by US Department of Energy.
Sample reorientation during the mixing time of a two-dimensional NMR exchange experiment allows the measurements of chemical shift tensors in powdered samples whose one-dimensional powder patterns are too complex to reliably analyze. This technique has the advantages that the chemical shift anisotropy has an unscaled representation, the artifacts caused by cross polarization, spin relaxation and partial sample orientation can be accounted for, the experiment is relatively easy to perform, and no spinning sidebands interfere. The method is useful for sorting out the principal values of overlapping chemical shift tensors.
1991
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.
1990
Two-Dimensional Chemical-Shift Tensor Correlation Spectroscopy. Analysis of Sensitivity and Optimal Measurement Directions
Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Journal of Magnetic Resonance, 86, 60-69, 1990. Funded by US Department of Energy.
Chemical-shift tensors can be determined from a single crystal placed in six or more orientations in a magnetic field. The sensitivity of this determination as a function of the selected crystal orientations is considered with a statistical figure of merit. A variety of configurations are examined, and it is found that the set of six orientations based on the vertices of an icosahedron optimizes the figure of merit and results in the most sensitive determination of the tensor. The relationship of these optimal orientations to those used previously in two-dimensional chemical-shift tensor correlation spectroscopy is discussed. It is shown that the high symmetry of the icosahedron simplifies the design on which the construction of a new sample orienting mechanism may be based.
1989
Two-Dimensional Chemical Shift Tensor Correlation Spectroscopy. Analysis of Sensitivity and Optimal Measurement Directions
Alderman, D.W.; Sherwood, M.H. and Grant, D.M.
Accepted for publication in J. Mag. Res., 1989. Funded by US Department of Energy.
Chemical shift tensors can be determined from a single crystal placed in six or more orientations in a magnetic field. The sensitivity of this determination as a function of the selected crystal orientations is considered with a statistical figure of merit. A variety of configurations are examined, and it is found that the set of six orientations based on the vertices of an icosahedron optimizes the figure of merit and results in the most sensitive determination of the tensor. The relationship of these optimal orientations to those used previously in two-dimensional chemical shift tensor correlation spectroscopy is discussed. It is shown that the high symmetry of the icosahedron simplifies the design on which the construction of a new sample orienting mechanism may be based.
1988
Efficient Stator/Rotor Assembly for Magic-Angle Spinning NMR
Jiang, Y.J.; Woolfenden, W.R.; Alderman, D.W.; Mayne, C.L.; Pugmire, R.J. and Grant, D.M.
Rev. SCI Instrum, 58, (5), 755-758, 1987. 4 pgs. Funded by US Department of Energy and Office of Basic Energy Sciences.
A newly designed stator assembly for cylindrical spinners used in magic-angle spinning nuclear-magnetic resonance experiments is described. Separate driving and bearing gas chambers allow variable and stable spinning speeds, and this design permits easy starting and stopping of the rotor. Isolation of the chambers is achieved with the application of pressure screws rather than O-rings or glue lines to avoid leakage at high gas pressures. The overall dimensions are optimal to facilitate easy assembly. Some significant modifications have been made to an earlier spinner design. These improvements give better efficiency and concentricity of the spinner. Applications are illustrated with carbon-13 cross polarization/magic angle spinning (CP/MAS) spectra carried out at different rotor spinning rates.
Carbon-13 Chemical Shift Tensors in Single-Crystal Methoxybenzenes
Carter, C.M.; Facelli, J.C.; Alderman, D.W.; Grant, D.M.; Dalley, N.K. and Wilson, B.E.
J. Chem. Soc. Faraday Trans. 1, 1988. Funded by US Department of Energy.
Using a two-dimensional NMR orientation correlation technique carbon-13 chemical-shift tensors have been measured in single crystals of 1.4-dimethoxybenzene. 1,3,5-trimethoxybenzene and 1,2,3-trimethosybenzene. The two-dimensional technique greatly extends the single-crystal method to materials with a much larger number of different carbon-13 resonance lines. The manner of dealing with chemically identical, but magnetically non-equivalent carbons in the unit cell is discussed. The X-ray structure of 1, 2, 3-trimethoxybenzene is reported for the first time, and a redetermination of the X-ray structure for 1.4-dimethoxybenzene is given. The principal values of all carbon-13 chemical-shift tensors in the three molecules and the orientation of their principal axes have been obtained. Using multiple regressional analysis the principal values of the tensors have been discussed in terms of additive substituent effects. AB initio calculations of the shielding tensors in anisole (methoxybenzene) and benzene were used to calculate substituent effects that agree closely with the parameters obtained from the regressional analysis.
Double Resonance Interferometry, Relaxation Modulated Coherence Transfer
Newmark, R.D.; Alderman, D.W. and Grant, D.M.
Submitted to Phys. Rev., 1988. Funded by US Department of Energy.
The double resonance interferometry experiment of Stoll, Vega and Vaughan is examined with numerical simulations employing the complete Redfield matrix. It is shown that their observed doublet asymmetry is a consequence of random field relaxation mechanisms. The relaxation pathways in the system are examined in detail to explain the coherence transfer that takes place during the SVV experiment. The simulations must include relaxation during the selective carbon-13 pulses in order to agree with the experimental results thereby indicating the need for care in using impulse approximations for selective pulses in some double resonance experiments.
Time-Domain Fitting and Director Reorientation of H-1-NMR Spectra of Molecules Dissolved in Liquid-Crystal Solvents
Heeschen, W.A.; Alderman, D.W. and Grant, D.M.
Submitted to J. Phys. Chem, 1988. Funded by US Department of Energy.
A time-domain fitting routine is described and applied to the analysis of a H-1-NMR spectrum of a solute in a liquid-crystalline solvent. Principles of the time-domain fitting technique are described in terms of data selection, baseline compensation and computational advantage over other fitting methods. Also, the use of fitting coefficients based on structural variation is described and found to provide rapid, reliable spectrum fits with physically reasonable structures. Solute ordering information is determined through interpretation of high-resolution spectra using director reorientation by the variable-angle sample spinning technique to provide a starting point for the fitting. Spectra from norbornadiene dissolved in the liquid crystal p-pentylphenyl 2-chloro-4-(p-pentylbenzoyloxy) benzoate are used to demonstrate the method. The resultant fit of the norbornadiene structure is compared to results obtained previously by other workers.