ABSTRACT
This chapter deals with the dipole-dipole (DD) relaxation, which has proven to be one of the most important sources for obtaining molecular dynamics information. The discussion on this relaxation mechanism is based on the Solomon equations, which, in turn, are based on rates of transitions between spin states. The chapter describes through the Solomon theory for the dipolar spin-lattice relaxation and its consequences. Good descriptions of dipolar interaction and dipolar relaxation can be found in the books by Abragam and by Levitt. The cross-relaxation phenomenon is, however, in principle always present in dipolar-relaxed systems, and the longitudinal magnetisations in a two-spin system do not follow the Bloch equations. The DD transition probabilities can be evaluated in terms of dipolar coupling constants and spectral densities corresponding to the angular variables. The chapter introduces the concept of intermolecular dipolar interactions and intermolecular dipolar relaxation, which implies that the relevant internuclear distances are not constant.
