Liu, F
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.
1989
Magnetization Preparation for Coupled Relaxation Studies Using J-Spectral Pulse Sequences (Technical Note)
Liu, F.; Mayne, C.L. and Grant, D.M.
J. Mag. Res., 84, 344, 1989. Funded by National Institutes of Health.
The accuracy with which one can determine spectral densities in a coupled relaxation experiment is strongly dependent on the nature of the non-equilibrium states from which the spin system is allowed to relax. Pulse sequences similar to those used to obtain 2D J-spectra have been used to prepare initial non-equilibrium states of an AX2 spin system. These preparations create a maximum perturbation for one of the magnetization modes which was poorly perturbed by other spin preparations. Inclusion of data obtained using the new preparation significantly reduces the marginal standard deviations of several of the spectral densities and reduces correlations among the spectral densities. Thus, a significant improvement in the structural and dynamical parameters extracted from the spectral densities is achieved.
Liu, F
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.
1989
Magnetization Preparation for Coupled Relaxation Studies Using J-Spectral Pulse Sequences (Technical Note)
Liu, F.; Mayne, C.L. and Grant, D.M.
J. Mag. Res., 84, 344, 1989. Funded by National Institutes of Health.
The accuracy with which one can determine spectral densities in a coupled relaxation experiment is strongly dependent on the nature of the non-equilibrium states from which the spin system is allowed to relax. Pulse sequences similar to those used to obtain 2D J-spectra have been used to prepare initial non-equilibrium states of an AX2 spin system. These preparations create a maximum perturbation for one of the magnetization modes which was poorly perturbed by other spin preparations. Inclusion of data obtained using the new preparation significantly reduces the marginal standard deviations of several of the spectral densities and reduces correlations among the spectral densities. Thus, a significant improvement in the structural and dynamical parameters extracted from the spectral densities is achieved.