ABSTRACT
This chapter deals with other multispin phenomena, which occur through interference of several relaxation mechanisms, involving the cross-correlation spectral densities. Cross-correlation effects can arise through interference between, for instance, the dipole-dipole (DD) interaction of a spin pair and the chemical shielding anisotropy (CSA) of one of the spins, or through interference between two dipole-dipole interactions. Furthermore, for spin 1/2 nuclei, measurement of cross-correlation between the dominating DD interaction and the CSA provides unique information about the interaction strength of the CSA and thus of the shielding anisotropy. Cross-correlation effects have been known from the early days of nuclear magnetic resonance, and in high-resolution Nuclear magnetic resonance they were primarily observed in double-resonance experiments. Several approaches to measuring cross-correlation have been proposed, both for small molecules and for biological macromolecules. Homonuclear cross-correlation effects can be measured in a similar way by applying a selective inversion pulse on one of the spins involved in the interaction.
