Double-quantum double-quantum MAS exchange NMR spectroscopy: Dipolar-coupled spin pairs as probes for slow molecular dynamics

By combining two dipolar double-quantum (DQ) MAS experiments, a homonuclear DO-DO MAS exchange experiment has been designed, which probes the reorientation of dipolar tensors and allows the observation of slow molecular dynamics, in particular the determination of reorientation angles and rates. The dipolar coupling between two distinct spins is used to generate a DQ coherence, and the orientation-dependent coupling is measured by means of the DQ MAS sideband pattern before and after a mixing time. In the course of a reduced three-dimensional experiment, the two DQ sideband patterns are correlated, resulting in a DO-DO sideband pattern which is sensitive to the reorientation angle. By referencing the DO-DO time signal, the information content of the pattern can be divided into the sidebands and the centerband, with the former reflecting only the moieties which have undergone a reorientation, and the latter predominantly containing contributions from moieties which have remained in, or returned to, their initial position. Hence, a single sideband pattern provides access to both the reorientation angle and the relative number of moieties subject to the motional process. As a first example, such DQ-DQ MAS experiments were performed on the C-13-C-13 spin pairs of an enriched poly(ethylene) sample. In its crystallites, the dynamics of a known chain-flip motion were investigated, yielding a C-13-C-13 reorientation angle of Delta theta (ij) = (70 +/- 5)degrees and an activation energy of E-A = (100 +/- 20) kJ mol(-1). (C) 2001 Academic Press.