Slow molecular motion detected in the NMR spectra of rotating solids

Abstract

We present an approach for detecting and analyzing slow molecular motions in solids based on changes in the zero order sideband of an inhomogeneously broadened spin system under magic angle spinning conditions. The effect is discussed theoretically from the standpoint of second order perturbation theory and the stochastic Liouville equation of Kubo, both for the case where the chemical shift is the main interaction. Experimental investigations of decamethylferrocene (resulting in Ea=3.3 kcal/mole) and hexamethyl benzene support the predicted trends. A five site jump model is used to interpret the results for decamethyl ferrocene, while the interpretation of the hexamethyl- benzene data is obfuscated by the presence of a chemical shift inequivalency presumably caused by a distortion of the aromatic ring above the phase transition (Tc=116 K).