Chemical Shift Anisotropy in Powdered Solids Studied by 2D FT NMR with Flipping of the Spinning Axis

Information about chemical shift anisotropy is usually difficult to extract from the spectrum of a nonspinning powdered sample because of the usually extensive overlap of the powder patterns from the chemically different sites in the molecule. Several types of experiments have been proposed to facilitate the measurement of chemical shift anisotropy (1-9). All have the common feature that the sample is rotated about the magic-angle axis (1-8) or an axis very close to the magic angle (9), either by rapid sample spinning (1-7, 9) or by rotating the sample in three discrete steps (8). We propose a new two-dimensional approach for obtaining the anisotropy information. In this new experiment, the spinning axis of the sample is flipped from 90 to 54.7” between the evolution and detection periods. The experiment appears to be widely applicable and has great promise for the study of complex samples. The experimental scheme is set out in Fig. 1. Cross polarization of, in our case, 13C nuclei is performed while the sample is spun about an axis that makes an angle of 90” with the static magnetic field. It can be shown (IO) that the powder anisotropy pattern that obtains under these conditions is reversed and collapsed to half the width of the static nonspinning case, but keeps the same shape. At the end of the evolution period (t,), the x component of the transverse 13C magnetization is stored along the z axis, parallel to the static magnetic field, by means of a 90,” 13C pulse. The orientation of the spinning axis of the sample is then changed to the magic angle. The sample is spun fast compared with the width of the anisotropy patterns, so that spinning sidebands have negligible intensities. A final 90 o 13C pulse rotates the z-stored 13C magnetization back into the transverse plane, where it precesses in the time domain, tZ, with the corresponding isotropic chemical shift frequencies. Cycling of the phase of the first 90 o 13C pulse alternately along +y and -y, together with adding and subtracting of the acquired data, is used to eliminate spurious signals. The detected isotropic spectrum, S(tl , F2) obtained by Fourier transformation with respect to t2, is modulated in amplitude with the frequencies existing during the evolution period, t,. Hence, the powder anisotropy information and the isotropic chemical shifts will appear in the F, dimension. Because of the amplitude modulation, a pure 2D absorption spectrum can be obtained by calculating the cosine Fourier transform, P(F,, FJ (II, 12).