Multi-dimensional H-1 NMR nuclear Overhauser spectroscopy under magic angle spinning: theory and application to elastomers

The process of cross-relaxation between different protons (nuclear Overhaus er effect) is investigated in soft solids by 2- and 3-dimensional NMR under the conditions of fast magic-angle spinning. The cross-relaxation rates ar e found to depend weakly on fast motions in the Larmor frequency range and strongly on slow motions of the order of the spinning frequency omega(R) Ex plicit expressions for the omega(R) dependent cross-relaxation rates are de rived for different motional models. These findings were tested experimenta lly on elastomers, i.e., on a crosslinking series of styrene-butadiene rubb ers where the cross-relaxation was studied as a function of omega(R) Short mixing times as are required for extracting the relaxation rates could be r ealized conveniently using a pulsed magnetic-field gradient for coherence p athway selection. As in solution NMR, relative couplings between chemically resolved spins can be determined from the peak intensities. By combining c ross-relaxation measurements with T-1 measurements, the distribution of cor relation times can be probed for slow and fast timescales, respectively. On ly the former were found to depend on the crosslink density.