The roto-translational diffusion of a molecule in a fluid

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Abstract
The coupling between the translational and rotational motions of a molecule in a fluid is investigated with the help of a diffusion equation for the self van Hove function G s. This equation is obtained from an exact balance equation for the microscopic density of molecules by introducing in the hydrodynamic limit phenomenological tensorial diffusion coefficients. Then substituting in this equation the expansion of G s on a complete basis set of rotational invariants we obtain an infinite system of coupled diffusion equations for the coefficients of the G s expansion. From the solution at wave vector k = 0 of this system we derive a new expression for the reorientational correlation times of a molecule.