Intermolecular Multiple Quantum Coherences in Liquids

In the early 1990s, the traditional framework of NMR spectroscopy was challenged through a series of simple experiments. The pulse sequences used consisted of a few RF pulses and a few gradient pulses, and the samples were mixtures of simple molecules. The spectra showed unexpected cross peaks between spins in different molecules. ( ) In order to explain these results, two basic assumptions had to be revisited: 1 the ( ) high-temperature approximation to the Boltzmann distribution at equilibrium, and 2 the cancellation of dipolar couplings in solution. A close look at the physics involved showed that correlations between spins in separate molecules exist even after a single pulse, and that dipolar couplings can make these correlations visible in the presence of gradient pulses. A comprehensive description of the effect is given here, and some present and future applications are discussed. 2000 John Wiley & Sons, Inc. Concepts Magn Reson 12: 396 409,