The application of MAS recoupling methods in the intermediate motional regime

We present investigations concerning the effect of molecular motions on the experimental timescale upon the recoupling of anisotropic interactions under magic-angle spinning conditions. An approach for the efficient simulation of spin dynamics occurring during complex pulse sequences, based on a linearization of the general solution of the stochastic Liouville-von Neumann equation, was developed. Using (13)C CSA recoupling of the methyl carbon in dimethylsulfon as a sample interaction, we observed a characteristic signal decay under recoupling upon entering the intermediate motional regime, which can be well described by an apparent transverse relaxation time, T(2)(rcpl). This quantity does not depend on the spinning frequency to a first approximation. Specific recoupling experiments, namely the measurement of tensor parameters by spinning sideband analysis, and the determination of rate constants with the CODEX experiment, are discussed with respect to possibilities and limits of their application in the intermediate motional regime. Important conclusions are drawn with regards to the limited applicability of popular recoupling methods like REDOR to samples exhibiting intermediate mobility.