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

The process of cross-relaxation between different protons (nuclear Overhauser effect) is investigated in soft solids by 2- and 3-dimensional NMR under the conditions of fast magic-angle spinning. The cross-relaxation rates are 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) Explicit expressions for the omega(R) dependent cross-relaxation rates are derived for different motional models. These findings were tested experimentally on elastomers, i.e., on a crosslinking series of styrene-butadiene rubbers 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 realized conveniently using a pulsed magnetic-field gradient for coherence pathway selection. As in solution NMR, relative couplings between chemically resolved spins can be determined from the peak intensities. By combining cross-relaxation measurements with T-1 measurements, the distribution of correlation times can be probed for slow and fast timescales, respectively. Only the former were found to depend on the crosslink density.