On nuclear spin-lattice relaxation due to hindered molecular reorientation

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Abstract
The nuclear spin-lattice relaxation rate 1/T1 is derived as a function of the angle xi between the axis of reorientation and the principal direction of the relaxation interaction, for a system of threefold symmetry which undergoes hindered molecular reorientation. The relaxation mechanisms considered are the anisotropic chemical shielding the nuclear quadrupole, and the nuclear dipole-dipole interactions. It is observed that 1/T1 is strongly dependent on xi and on the interaction anisotropy. In addition the motional conditions are derived under which the Overhauser signals due to the dipolar coupling between unlike spins, will change sign and where the usually non-exponential relaxation becomes exponential for all practical purposes, thereby substantially easing the extraction of motional correlation times.