Some Structural Aspects of the Rotational and Translational Diffusion in Liquids

An interconnection between the rotational correlation time of a molecule and the structure of the liquid is demonstrated. For this purpose the angular velocity correlation function is constructed by taking account of fluctuating effective moments of inertia of aggregates due to molecular association. From this correlation function the rotational diffusion coefficient or rotational correlation time is obtained. The probabilities for the occurence of various aggregates which appear in this treatment are expressed in terms of integrals, containing the orientation dependent pair distribution function and a function describing the chance that the aggregate is sufficiently long‐lived. The reorientation time increases with increasing degree of structure. A numerical example for water is presented. The situation in mixtures and the occurence of intramolecular motions is discussed. The analogous treatment for the translational self‐diffusion coefficient is sketched.