Transverse relaxation of dipolar coupled spin systems under rf irradiation: Detecting motions in solids

Abstract

The spin–spin relaxation time T2 for an S spin dipolar coupled to an unlike I spin under conditions of random isotropic rotational motion and I spin rf decoupling is calculated. The results of the analysis form the basis for a method to detect and analyze rotational motions in solids. The technique is sensitive to motions which occur on the time scale of the rotating-frame Larmor frequency of the applied rf field and is presented in the context of a variable temperature magic-angle sample-spinning experiment where the S spins are observed and the I spins are decoupled. The previously well characterized molecules hexamethylbenzene, hexamethylethane, and adamantane are studied (S=13C and I=1H), and the resulting motional parameters are found to be in good agreement with previous results. As well, the theoretically predicted dependence of T2 on the strength of the decoupling field is experimentally borne out. Finally, the question of resolution of heteronuclear coupled spin systems and the role of decoupling is addressed.