ROTATIONAL DIFFUSION KINETICS OF POLAR SOLUTES IN HEXAMETHYLPHOSPHORAMIDE WATER-SYSTEMS

The rotational reorientation times (tau(r)) of three kinds of dyes-cre syl violet (a monocation), nile red (neutral but polar), and resorufin (a monoanion) were measured in a series of binary mixtures of hexamet hylphosphoramide (HMPA)-water at 298 K using the picosecond fluorescen ce depolarization spectroscopy. A hook-type profile of tau(r) vs eta w as observed for all three dyes. The contribution due to the dielectric friction is minimal and is irrelevant in explaining the rotational dy namics of nile red. The dielectric friction correction is modest for c resyl violet and resorufin. The solvation effects in the binary mixtur e are dominant for understanding the rotational dynamics of cresyl vio let. The microscopic structure features of the binary mixture are brou ght out by the study of resorufin.