THE BOUNDARY BETWEEN LIQUID-LIKE AND SOLID-LIKE BEHAVIOR IN MAGNETIC-RESONANCE

Recent experimental work in two-dimensional solution NMR (nuclear magn etic resonance) has demonstrated anomalous cross-peaks and additional resonances due to dipolar couplings between distant nuclei. These spec tra have been analyzed either classically, using Bloch equations which include a mean-field approximation to the demagnetizing field, or qua ntum mechanically, using a full density matrix picture which shows tha t the peaks correspond to intermolecular multiple-quantum coherences ( iMQCs). Here we use a density matrix treatment to predict intensities in solution for dipolar effects conventionally seen in solids; we also explore in detail the fundamental differences between dipolar effects in solids and liquids. For example, even though polarization transfer via the dipolar Hamiltonian in solution is not possible, indirect det ection with substantial signal enhancement is possible. We find that, even for high-gamma nuclei such as H-1 or He-3, solidlike dipolar effe cts are quite small unless the diffusion constant is roughly one milli on times smaller than that of water-which means that deviations betwee n the quantum and classical treatments are barely observable in soluti on NMR, and that even solid 3He has liquidlike dipolar effects in agre ement with experiment. However, the dipolar correlation function has a n extremely unusual functional form-the long time falloff is proportio nal to t(-3/2), not the exponential one commonly encounters. Because o f this long falloff, solidlike dipolar effects can be substantial in s olution electron spin resonance, and the classical picture of the dema gnetizing field would fail in that case. (C) 1998 American Institute o f Physics. [S0021-9606(98)01304-X].