Most attention has been given to the dipole-dipole (DD), chemical shift anisotropy (CSA) and random field relaxation mechanisms . More unusual relaxation mechanisms such as nuclear spin-rotation and scalar relaxation of the second kind have also been identified . Generally speaking, the efficiency of a relaxation mechanism is roughly proportional to the rotational correlation time τ c for the random tumbling of the molecules in solution. This suggests that small molecules, which tumble more rapidly in solution and have small values of τ c , tend to have the longest LLS lifetimes. In most cases, that is also the observed behaviour.