NMR measurements of scalar-coupling distributions in disordered solids
The measurement of scalar (J) couplings by solid-state NMR is a field of great interest, since this interaction is a rich source of local structural information, complementary to dipolar and chemical shift interactions. Here, we first demonstrate that J-coupling distributions exist and can be observed in disordered solids, as illustrated with the observation of a pair-specific distribution of (2)J(P-31-N-P-31) couplings in a bis-phosphino amine, and we investigate the potential effects of such distributions on the measurement of average J-coupling constants. Second, we show that the measurement of two-dimensional (2D) distributions of J-couplings provides a much richer probe of local structural disorder than one-dimensional distributions, and we introduce new methods that provide different (selective or non-selective) ways of measuring 2D J distributions in a wide range of disordered systems. These methods are finally applied to a slightly disordered polymorphic sample of fully C-13-enriched cellulose, and then to the bis-phosphino amine sample, from which 2D (2)j(PP)-coupling distributions are clearly identified and interpreted.