Cross-correlations in multispin relaxation

Whenever there is more than one mechanism for the relaxation of a spin, there can be cross-terms between these mechanisms. These cross-terms, known as cross-correlations affect both the longitudinal and the transverse relaxation of a spin, as well as the nuclear Overhauser effect (NOE) to the other spins. The major effect of these cross-terms is a differential effect, also known as multiplet effect between various transitions of a spin. The detection of the multiplet effect therefore requires that the various transitions be resolved by J-coupling. A smaller effect is a second-order net effect, which also can influence the various relaxation measurements. This article is a pedagogic review of cross-correlation effects in longitudinal and transverse relaxation and NOE experiments. Representative experiments demonstrating the presence of cross-correlation in the longitudinal and transverse relaxation of a spin are also discussed. (C) 1996 John Wiley & Sons, Inc.