The Fokker-Planck-Langevin model for rotational brownian motion. IV. Asymmetric top molecules

Abstract

The general series expansion for the joint angular velocity-orientation conditional probability density for a fluid composed of asymmetric top molecules is derived, and expressions for the reorientational correlation functions, correlation times, spectral densities, and the correlation times relevant to magnetic relaxation via spin-rotation interactions are presented. The reorientational correlation times and spin-rotation functions computed using this Fokker-Planck-Langevin (FPL) model with isotropic friction tensor (τ x = τ y = τ z ) are compared with the corresponding times and functions computed with the J-diffusion limit of the extended diffusion (ED J) model. The models are found to predict significantly different behaviour only in the regime where free rotation and precessional effects become important. The reorientation self- and cross-correlation times are shown to follow Hubbard relations in the limit of short τ x , τ y , τ z . Numerical calculations of the FPL reorientational correlation functions, correlation times and spin-rotation functions show that these properties are sensitive to the anisotropies in the friction tensor in the rotational Langevin equation.