The short-time dynamics of molecular reorientation in liquids. II. The microscopic mechanism of rotational friction

At short times, the dynamics of the rotational relaxation of linear molecules dissolved in liquids is governed by the instantaneous rotational friction, a quantity one can specify in complete molecular detail for each liquid configuration. Having the ability to construct such a friction is not only useful for the insight it provides into rotational dynamics, it means that it is possible to think about the superficially very different processes of rotational relaxation, vibrational population relaxation and solvation in a common language. In particular, the ability to understand the friction in molecular terms allows us to compare the actual solvent molecules participating and the actual solvent motions involved in all of these relaxation processes. In this paper we carry out a detailed study of the rotational friction felt by a homonuclear diatomic molecule dissolved in an atomic fluid, contrasting the results for a variety of solute sizes and thermodynamic states. We find remarkable levels of similarity among all three kinds of relaxation. While there are some detailed differences in the geometry of the relevant solvent motions, all three processes seem to be controlled by a small number of nearby solvents. Possibly as a result, the influence spectra (the spectral densities) of all three are virtually identical. The invariance of these findings, and indeed of the mechanistic details, to solute size and thermodynamic conditions suggests that there is a real universality to solution dynamics that comes into play when sharply varying forces are involved. (C) 2000 American Institute of Physics. [S0021-9606(00)50317-1].