Pseudo Rotary Resonance Relaxation Dispersion Effects in Isotropic Samples

Abstract. Enhanced transverse relaxation near rotary-resonance conditions is a well-documented effect for anisotropic solid samples undergoing magic-angle spinning (MAS). It is a surprising behavior for rotating liquids, in which first-order anisotropic interactions are averaged at a much faster timescale as compared with the spinning frequency. Here we report measurements of ¹³C transverse relaxation under spin lock for spinning samples of both polybutadiene rubber and polyethylene glycol solution. Maxima in the relaxation rates are observed when the spin-lock frequency matches one or two times the MAS rate. Through simulations, we qualitatively describe the appearance of this effect, which can be explained by time dependence caused by sample rotation and an inhomogeneous rf-field distribution. Consideration of this effect is important for MAS experiments based on rotary-resonance conditions, and motivates the design of new MAS coils with improved rf-field homogeneity.