Off-axis sample spinning in the slow-spinning regime

Abstract

The past few years have witnessed an increasing interest in the study of solids by means of high-resolution NMR techniques (I, 2). Of the various approaches that have been developed, the combination of polarization transfer and dipolar decoupling (3) together with magic-angle sample spinning (MASS) (4, 5) has become a popular tool in the study of a wide variety of amorphous and polycrystalline samples (6). In this case, heteronuclear dipolar broadening is attenuated by irradiation of the proton spectrum by a strong resonant rf field, and residual broadening due to the chemicalshift anisotropy of the dilute spins is removed by spinning the sample around the magic angle. When the frequency, UR, is fast compared to the shift anisotropy (in hertz), then a high-resolution “liquid-like” spectrum, devoid of all information other than the isotropic chemical shifts, is observed. Because these experiments lead to a suppression of the chemical-shift anisotropy, they have stimulated a search for methods to reintroduce this information in a manner consistent with the goal of high resolution. The two methods most frequently discussed for this purpose involve application of a train of pulses synchronized with the sample spinning (7-11) or analysis of the sideband intensities in the slowspinning regime (12, 13). The former approach results in a distorted (but calculatable) powder lineshape or sideband pattern which must be resolved in some cases by twodimensional techniques. In the latter case, the spinning speed is reduced so that the condition VR AC this approach leads to a centerband consisting of a powder pattern of much reduced breadth and which changes sign as the spinning angle moves from one side of 54.7” to the other. However, in the limit vR < AU we have observed some unexpected results. In particular, the centerband and sidebands consist of