Hu, JZ
1997
Dynamic Nuclear Polarization of Nitrogen-15 Benzamide
Hu, J.Z.; Zhou, J.; Yang, B.; Li, L.; Qiu, J.; Ye, C.; Solum, M.S.; Wind, R.A.; Pugmire, R.J. and Grant, D.M.
SOLID STATE: Nuclear Magnetic Resonance, 8:129-37(1997). Funded by US Department of Energy/Basic Energy Services and ACERC.
A N-15 dynamic nuclear polarization (DNP) experiment is reported in which a N-15 DNP enhancement factor of approximately 2.6 x 10² is obtained on free radical doped samples of 99% N-15 labeled benazmide. The free radicals BDPA (1:1 complex of alpha, gamma-bisdiphenylene-beta-phenylallyl with benzene) and DDPH (2,2 -Di (4 - tert - octylphenyl) -1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T1 values of the samples. The combination is solids of a very low natural abundance, 0.37%, a small gyromagnetic ration, and a log spin - lattice relaxation time for N-15 nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the N-15 DNP experiment on samples containing free electrons.
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 and Hopping Experiments
Hu, J.Z.; Wang, W. and Pugmire, R.J.
Encyclopedia of NMR, J. Wiley & Sons, Ltd., 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.
It is well known that in a solid, the chemical shift of a nucleus is a function of molecular orientation with respect to the external magnetic field. This phenomenon is described as the chemical shift anisotropy (CSA), and it is directly related to the local electronic structure of the nucleus. The principal values of the CSA as well as their orientation in the molecular frame can be obtained through a single crystal study (see "Chemical Shift Measurement by Single Crystal Techniques"). However, for the majority of compounds, the difficulties associated with growing a single crystal of sufficient size limits the application of current single crystal methods. In a powder sample the orientation information is lost because of the random distribution of the crystallites. However, the principal values, which are very useful in characterizing the structure of a molecule, are still available in the powder pattern obtained from a stationary or slowly spinning sample when the molecule has few enough unique nuclei that the spectrum can be interpreted. Unfortunately, overlap of several broad powder patterns often prevents the spectral separation necessary for individual identification and measurement.
In an effort to address this problem of spectral overlap, many 2-D techniques have been developed to obtain a 2D spectrum with an isotropic shift projection along one dimension and a stationary or slow-spinning-sideband powder pattern along the other (see "Chemical Shift Tensors"). One of the first techniques developed was the magic angle hopping (MAH) experiment of Bax et. al. By successively "hopping" the sample 120° about an axis at the magic angle, an isotropic shift dimension is obtained since the average of the resonance frequencies at the three orthogonal positions is the isotropic shift.
An analog of the MAH experiment employing continuous slow rotation of the sample has recently been demonstrated by Gan. Gan's elegant technique uses pulses spaced at 1/3 of the rotor period to obtain the isotropic shift evolution. We call Gan's experimental technique the Magic Angle Turning (MAT) experiment because of the very slow rotation involved. Significant improvements in the experimental details have been made to optimize these two experiments.
In this article, a simple theory is given to describe the MAH experiment and the most recent version of the MAT experiments together with typical experimental results included to show the basic principals and the power of these two related methods.
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.
Solid State C-13 NMR, X-Ray and Quantum Mechanical Studies of the Carbon Chemical Shift Tensors of P-Tolyl Ether
Facelli, J.C.; Hu, J.Z.; Orendt, A.M.; Arif, A.M.; Pugmire, R.J. and Grant, D.M.
Journal of Physical Chemistry, 1994 (in press). Funded by US Department of Energy and ACERC.
This paper presents a detailed study of the principal components of the C-13 chemical shift tensors in p-tolyl ether. The tensor components of a relative large number of carbon atoms are measured by using the two-dimensional magic angle turning (MAT) technique that allows for the determination of the principal components of the chemical shift tensors in powders. Theoretical calculations of the C-13 chemical shieldings, using the X-ray molecular geometry, are used to assign the NMR resonances to individual carbon nuclei. The principal values of the chemical shift tensors permit assignments that would be unreliable if only the isotropic shift information is used. The chemical shift tensors of the carbons directly attached to the oxygen atom are very sensitive to the structural and electronic properties of the ether linkage. The combination of the C-13 MAT experiment and theoretical chemical shieldings proves to be important in the study of electronic properties and molecular structure.
Measurement of C-13 Chemical Shift Tensor Principal Values with a Magical Angle Turning Experiment
Hu, J.Z.; Orendt, A.M.; Alderman, D.W.; Pugmire, R.J.; Ye, C. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 5:181, 1994. Funded by US Department of Energy and ACERC.
The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of C-13 chemical shift tensors in powdered solids. A large-volume MAT prove with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively, can enhance measurements of the principal values of chemical shift tensors in complex compounds. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the C-13 principal values in 2,3-dimethlnaphthalene and Pocohontas coal are reported at typical examples.
The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment
Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
ACS Preprints, Division of Fuel Chemistry, 39:8-112, 1994. Funded by Pittsburg Energy Technology Center, US Department of Energy and ACERC.
The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high-resolution spectra in complex solids such as coal. MAS narrows the chemical shift anisotropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values. In and effort to address this problem of spectral overlap, many 2D techniques have been developed to simultaneously obtain the dispersion by isotropic shift, such as produced by MAS, in one dimension and the tensorial information as a separate powder patters in the second dimension. A very successful technique is the slow spinning modification of the magic angle hopping experiment recently proposed by Gan, which we call the Magic Angle Turning (MAT) experiment. This experiment has a number of advantages over earlier 2D methods. The use of very slow spinning speeds (<50 Hz) favors the quantitative polarization of all carbons and allows the use of a large volume sample rotor resulting in a typical 2D spectrum acquisition requiring less than 24 hours. The mechanical device for slow spinning is very stable and high resolution in the isotropic chemical shift dimension can be easily obtained. The MAT experiment could be done on a suitably stable MAS probe. The only disadvantages of the original MAT experiment is that data acquisition starts right after the last pulse, causing distortion in the evolution dimension (the second dimension) even if a delay as short as 20 ms is used.
In this paper, a triple-echo MAT sequence, previously described, is employed which improves the 2D baseline. Two additional experiments, using short contact times and dipolar dephasing techniques, are also employed to further separate the powder patterns of protonated and nonprotonated carbons in complex compounds. Experimental results on representative model compounds as well as coals are presented in this paper.
1993
Improvements to the Magic Angle Hopping Experiment
Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 2:235-243, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.
Several improvements to the magic angle hopping experiment first introduced by Bax et al. (J. Magn. Reason., 52 (1983) 147 are presented. A dc servo motor driven sample hopping mechanism which requires less than 60 ms to accomplish a 120º sample rotation is described. Modifications to the data acquisition process, including starting the acquisition period immediately after the second hop and acquiring a hypercomplex data set, are also presented. Principal values of the C-13 chemical shielding tensor are measured for 1,2,3-trimethoxybenzene and 2,6-dimethoxynaphthalene.
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.
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
Selective Saturation and Inversion of Multiple Resonances in High-Resolution Solid-State C-13 Experiments Using Slow Spinning CP/MAS and Tailored Dante Pulse Sequences
Hu, J.Z.; Pugmire, R.J.; Orendt, A.M.; Grant, D.M. and Ye, C.
Solid State Nuclear Magnetic Resonance, 1:185-195, 1992. [Also included in ACS Division of Fuel Chemistry Preprints, 37(2):646-659, 1992 (203rd ACS National Meeting, San Francisco, CA, April 1992)]. Funded by US Department of Energy and Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction.
Taking advantage of the long C-13 T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in C-13 CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90% DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of his method, including the simplification of the measurement of the principal values of the C-13 chemical shift tensor under slow MAD conditions are described. The determination of the aromaticity of coal using a relatively slow MAS spinning rate is also described.
Hu, JZ
1997
Dynamic Nuclear Polarization of Nitrogen-15 Benzamide
Hu, J.Z.; Zhou, J.; Yang, B.; Li, L.; Qiu, J.; Ye, C.; Solum, M.S.; Wind, R.A.; Pugmire, R.J. and Grant, D.M.
SOLID STATE: Nuclear Magnetic Resonance, 8:129-37(1997). Funded by US Department of Energy/Basic Energy Services and ACERC.
A N-15 dynamic nuclear polarization (DNP) experiment is reported in which a N-15 DNP enhancement factor of approximately 2.6 x 10² is obtained on free radical doped samples of 99% N-15 labeled benazmide. The free radicals BDPA (1:1 complex of alpha, gamma-bisdiphenylene-beta-phenylallyl with benzene) and DDPH (2,2 -Di (4 - tert - octylphenyl) -1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T1 values of the samples. The combination is solids of a very low natural abundance, 0.37%, a small gyromagnetic ration, and a log spin - lattice relaxation time for N-15 nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the N-15 DNP experiment on samples containing free electrons.
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 and Hopping Experiments
Hu, J.Z.; Wang, W. and Pugmire, R.J.
Encyclopedia of NMR, J. Wiley & Sons, Ltd., 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.
It is well known that in a solid, the chemical shift of a nucleus is a function of molecular orientation with respect to the external magnetic field. This phenomenon is described as the chemical shift anisotropy (CSA), and it is directly related to the local electronic structure of the nucleus. The principal values of the CSA as well as their orientation in the molecular frame can be obtained through a single crystal study (see "Chemical Shift Measurement by Single Crystal Techniques"). However, for the majority of compounds, the difficulties associated with growing a single crystal of sufficient size limits the application of current single crystal methods. In a powder sample the orientation information is lost because of the random distribution of the crystallites. However, the principal values, which are very useful in characterizing the structure of a molecule, are still available in the powder pattern obtained from a stationary or slowly spinning sample when the molecule has few enough unique nuclei that the spectrum can be interpreted. Unfortunately, overlap of several broad powder patterns often prevents the spectral separation necessary for individual identification and measurement.
In an effort to address this problem of spectral overlap, many 2-D techniques have been developed to obtain a 2D spectrum with an isotropic shift projection along one dimension and a stationary or slow-spinning-sideband powder pattern along the other (see "Chemical Shift Tensors"). One of the first techniques developed was the magic angle hopping (MAH) experiment of Bax et. al. By successively "hopping" the sample 120° about an axis at the magic angle, an isotropic shift dimension is obtained since the average of the resonance frequencies at the three orthogonal positions is the isotropic shift.
An analog of the MAH experiment employing continuous slow rotation of the sample has recently been demonstrated by Gan. Gan's elegant technique uses pulses spaced at 1/3 of the rotor period to obtain the isotropic shift evolution. We call Gan's experimental technique the Magic Angle Turning (MAT) experiment because of the very slow rotation involved. Significant improvements in the experimental details have been made to optimize these two experiments.
In this article, a simple theory is given to describe the MAH experiment and the most recent version of the MAT experiments together with typical experimental results included to show the basic principals and the power of these two related methods.
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.
Solid State C-13 NMR, X-Ray and Quantum Mechanical Studies of the Carbon Chemical Shift Tensors of P-Tolyl Ether
Facelli, J.C.; Hu, J.Z.; Orendt, A.M.; Arif, A.M.; Pugmire, R.J. and Grant, D.M.
Journal of Physical Chemistry, 1994 (in press). Funded by US Department of Energy and ACERC.
This paper presents a detailed study of the principal components of the C-13 chemical shift tensors in p-tolyl ether. The tensor components of a relative large number of carbon atoms are measured by using the two-dimensional magic angle turning (MAT) technique that allows for the determination of the principal components of the chemical shift tensors in powders. Theoretical calculations of the C-13 chemical shieldings, using the X-ray molecular geometry, are used to assign the NMR resonances to individual carbon nuclei. The principal values of the chemical shift tensors permit assignments that would be unreliable if only the isotropic shift information is used. The chemical shift tensors of the carbons directly attached to the oxygen atom are very sensitive to the structural and electronic properties of the ether linkage. The combination of the C-13 MAT experiment and theoretical chemical shieldings proves to be important in the study of electronic properties and molecular structure.
Measurement of C-13 Chemical Shift Tensor Principal Values with a Magical Angle Turning Experiment
Hu, J.Z.; Orendt, A.M.; Alderman, D.W.; Pugmire, R.J.; Ye, C. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 5:181, 1994. Funded by US Department of Energy and ACERC.
The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of C-13 chemical shift tensors in powdered solids. A large-volume MAT prove with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively, can enhance measurements of the principal values of chemical shift tensors in complex compounds. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the C-13 principal values in 2,3-dimethlnaphthalene and Pocohontas coal are reported at typical examples.
The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment
Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
ACS Preprints, Division of Fuel Chemistry, 39:8-112, 1994. Funded by Pittsburg Energy Technology Center, US Department of Energy and ACERC.
The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high-resolution spectra in complex solids such as coal. MAS narrows the chemical shift anisotropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values. In and effort to address this problem of spectral overlap, many 2D techniques have been developed to simultaneously obtain the dispersion by isotropic shift, such as produced by MAS, in one dimension and the tensorial information as a separate powder patters in the second dimension. A very successful technique is the slow spinning modification of the magic angle hopping experiment recently proposed by Gan, which we call the Magic Angle Turning (MAT) experiment. This experiment has a number of advantages over earlier 2D methods. The use of very slow spinning speeds (<50 Hz) favors the quantitative polarization of all carbons and allows the use of a large volume sample rotor resulting in a typical 2D spectrum acquisition requiring less than 24 hours. The mechanical device for slow spinning is very stable and high resolution in the isotropic chemical shift dimension can be easily obtained. The MAT experiment could be done on a suitably stable MAS probe. The only disadvantages of the original MAT experiment is that data acquisition starts right after the last pulse, causing distortion in the evolution dimension (the second dimension) even if a delay as short as 20 ms is used.
In this paper, a triple-echo MAT sequence, previously described, is employed which improves the 2D baseline. Two additional experiments, using short contact times and dipolar dephasing techniques, are also employed to further separate the powder patterns of protonated and nonprotonated carbons in complex compounds. Experimental results on representative model compounds as well as coals are presented in this paper.
1993
Improvements to the Magic Angle Hopping Experiment
Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 2:235-243, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.
Several improvements to the magic angle hopping experiment first introduced by Bax et al. (J. Magn. Reason., 52 (1983) 147 are presented. A dc servo motor driven sample hopping mechanism which requires less than 60 ms to accomplish a 120º sample rotation is described. Modifications to the data acquisition process, including starting the acquisition period immediately after the second hop and acquiring a hypercomplex data set, are also presented. Principal values of the C-13 chemical shielding tensor are measured for 1,2,3-trimethoxybenzene and 2,6-dimethoxynaphthalene.
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.
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
Selective Saturation and Inversion of Multiple Resonances in High-Resolution Solid-State C-13 Experiments Using Slow Spinning CP/MAS and Tailored Dante Pulse Sequences
Hu, J.Z.; Pugmire, R.J.; Orendt, A.M.; Grant, D.M. and Ye, C.
Solid State Nuclear Magnetic Resonance, 1:185-195, 1992. [Also included in ACS Division of Fuel Chemistry Preprints, 37(2):646-659, 1992 (203rd ACS National Meeting, San Francisco, CA, April 1992)]. Funded by US Department of Energy and Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction.
Taking advantage of the long C-13 T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in C-13 CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90% DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of his method, including the simplification of the measurement of the principal values of the C-13 chemical shift tensor under slow MAD conditions are described. The determination of the aromaticity of coal using a relatively slow MAS spinning rate is also described.