Methyl NMR relaxation: The effects of spin rotation and chemical shift anisotropy mechanisms

Abstract

The formalism developed by the author in a previous publication for the calculation of methyl NMR relaxation by intramolecular dipolar interactions is applied to the spin rotation and chemical shift anisotropy mechanisms. The spin rotation and chemical shielding tensors are put into spherical form to allow the Hamiltonians for the above mechanisms to be developed in terms of spherical tensors. Because the calculations are directed towards elucidating the behavior of a methyl group attached to a macromolecule in solution, only the methyl internal rotation is assumed to contribute through the spin rotation mechanism to the overall relaxation. The importance of properly taking the spin rotation contribution into account, even when the dipolar mechanism is dominant, is demonstrated. A simple picture for the occurrence of correlations between the dipolar and chemical shift anisotropy mechanisms is presented, and the relaxation matrices, including the cross terms, are given for these two mechanisms. The forms of the relaxation matrices indicate that the effect due to the cross terms is small, and this contention is supported by calculations performed for a trifluoromethyl group.