Rotational Diffusion of Symmetric Top Molecules in Liquids

Abstract

The extended M‐ and J‐diffusion models previously proposed to describe the reorientation of linear and spherical molecules are used to treat liquids composed of symmetric top molecules. The reorientational correlation functions and their Fourier transforms are determined and shown to reduce, in the J‐diffusion model in the limit of very short angular momentum correlation times τ_J, to the results obtained by solution of the anisotropic diffusion equation. The applicability of the diffusion equation results, outside the domain of validity of the diffusion equation, are examined in detail. Magnetic spin relaxation due to motional modulation of spin‐rotational interactions is also considered within the framework of the extended diffusion models, and the inertial motion of the molecules and the precessional motion of the angular momentum are shown to influence the contributions due to the modulation of the anisotropic part of the spin‐rotational interactions.