MAGIC-ANGLE-TURNING EXPERIMENTS FOR MEASURING CHEMICAL-SHIFT-TENSOR PRINCIPAL VALUES IN POWDERED SOLIDS

The magic-angle-turning (MAT) technique introduced by Gan employs slow (approximately 30 Hz) rotation of a powdered sample at the magic angl e, in concert with pulses synchronized to 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 w hich, 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 pr ecludes the use of 1D methods. Previous MAT implementations are review ed, and it is shown how a new phase-corrected MAT (PHORMAT) pulse sequ ence overcomes many of their limitations. This new pulse sequence prod uces a spinning-sideband-free isotropic-shift spectrum directly as a p rojection 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. Fla t 2D baseplanes 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 o n absolute rotor-frequency stability is described. The PHORMAT spectru m of methyl-alpha-D-glucopyranoside is presented. The data are analyze d with an emphasis on the quantitative accuracy of the experiment in m easuring chemical-shift-tensor principal values and determining the re lative number of spins of each type present. The FID data from the spe ctrometer acquisition are fitted with numerical simulations that emplo y a banded-matrix method for calculating spinning-sideband amplitudes. The chemical-shift principal values. measured in methyl-a-D-glucopyra noside with the PHORMAT method, are compared with those from a single- crystal determination of the full chemical-shift tensors. The two meas urements differ by an RMS-average distance of only 0.57 ppm. (C) 1995 Academic Press, Inc.