Spatially selective excitation in solid-state NMR by off-resonance spin tipping

Abstract

A simple and efficient technique for spatially selective excitation of proton NMR in solids is described. The technique is based on selective saturation resulting from repeated off resonance RF tipping of protons under an applied field gradient, with a suitable dipolar dephasing interval inserted between the tippings. The tip angle about the effective field is adjusted to integral multiples of 2π at the magic angle in the rotating frame. This permits the selective preparation of longitudinal magnetization of those protons which satisfy the magic-angle condition, without significant loss of magnetization due to the dipolar couplings. Computer simulations illustrate the applicability of the technique. An experimental example of the excitation profile is presented by directly monitoring the selectively prepared longitudinal magnetization with the magic-sandwich-echo high-resolution imaging sequence.