Reorientation of small molecules and anions in solution studied by resonance enhanced dynamic Rayleigh scattering

Abstract

Resonance enhanced dynamic depolarized Rayleigh scattering is used to measure reorientation times of two nitrophenolate anions in millimolar aqueous and ethanolic solutions. Reorientation times of the corresponding neutral molecules in 0.01 to 0.1 M solutions are also reported. The reorientation time of 3,4-dinitrophenolate anion has a linear viscosity dependence when the viscosity is varied by varying temperature. The ratio of reorientation time to viscosity C is the same for neutral 3,4-dinitrophenol and its anion in water to within experimental error. In ethanol the C values for both neutral molecule and anion are much larger than the corresponding water values and the anion C value depends upon counterion. This final observation suggests ion pairing occurs in these solutions. The spectra of picrate salts in ethanol clearly show the presence of a second species (ion pair). The picrate anion C value in ethanol determined from the broad component of a two Lorentizain fit is slightly larger than the corresponding water value. The results from this work are compared with reorientation times of small anions reported in the literature, most of which were obtained using picosecond absorption and fluorescence techniques. Ion pairing has not been properly accounted for in many of these experiments, preventing futher conclusions concerning solute–solvent interactions. Some suggestions for futher experiments are given.