INTERMOLECULAR MULTIPLE-QUANTUM COHERENCES AND CROSS CORRELATIONS IN SOLUTION NUCLEAR-MAGNETIC-RESONANCE

It was recently reported that multiple-quantum NMR coherences could ap parently be observed in water and other concentrated samples, in direc t violation of established theory. These results were previously expla ined in a dressed-state framework as manifestations of the coupling be tween the spins and the coil (quantized radiation damping). Here we pr ovide details of previously communicated experimental explorations of these effects [J. Chem. Phys. 96, 1659 (1992)], and we extend these re sults to multicomponent samples. We observe cross peaks between indepe ndent molecules in solution in two-dimensional experiments, including spectra with multiple-quantum coherence transfer echoes; we also demon strate coherence transfer between solvent and (dilute) solute molecule s. However, we show that these intermolecular cross peaks are induced by a mechanism which is local in nature, and thus radiation damping (e ither classical or quantized) cannot provide the bulk of the explanati on for their occurrence. Simulations and analytical results show that the dipolar demagnetizing field can account for many of these surprisi ng effects, although a complete picture must be more complex.