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). We report transverse signal decay associated with rotary resonance conditions for rotating liquids, a surprising observation, since first-order anisotropic interactions are averaged at a much faster timescale compared with the spinning frequency. We report measurements of 13C and 1H signal intensities under spin lock for spinning samples of polybutadiene rubber, polyethylene glycol solution, and 99.96 % D2O. A drastic reduction in spin-lock signal intensities is observed when the spin-lock frequency matches 1 or 2 times the MAS rate. In addition, oscillations of the signal are observed, consistent with a coherent origin of the effect, a pseudo rotary resonance relaxation dispersion (pseudo-RRD). Through simulations, we qualitatively describe the appearance of pseudo-RRD, which can be explained by time dependence caused by sample rotation and an inhomogeneous field, the origin of which is an instrumental imperfection. Consideration of this effect is important for MAS experiments based on rotary resonance conditions and motivates the design of new MAS coils with improved radio frequency (RF)-field homogeneity.