Intermolecular multiple quantum coherences in liquids

In the early 1990s, the traditional framework of NMR spectroscopy was chall enged through a series of simple experiments. The pulse sequences used cons isted of a few RF pulses and a few gradient pulses, and the samples were mi xtures of simple molecules. The spectra showed unexpected cross peaks betwe en spins in different molecules. In order to explain these results, two bas ic assumptions had to be revisited: (1) the high-temperature approximation to the Boltzmann distribution at equilibrium, and (2) the cancellation of d ipolar couplings in solution. A close look at the physics involved showed t hat correlations between spins in separate molecules exist even after a sin gle pulse, and that dipolar couplings can make these correlations visible i n the presence of gradient pulses. A comprehensive description of the effec t is given here, and some present and future applications are discussed. (C ) 2000 John Wiley & Sons, Inc.