Calculations on ionic solvation. VII. The free energy of solvation of ions calculated from various local solvent dielectric constant-distance functions

Abstract

Calculations on the electrostatic free energy of solvation of Rb⁺ (and also Br^-) in acetonitrile have been carried out by using various functions that relate the local solvent dielectric constant to distance from the centre of an ion immersed in the solvent. A simple one-step dielectric constant-distance function yields a calculated value in good agreement with the experimentally observed value; this agreement does not depend on any arbitrary single-ion convention. The new dielectric constant profile suggested by Stiles leads generally to too negative a calculated value, as does the Born equation, but an amended profile of similar mathematical form to the function used by Stiles also leads to good agreement between calculated and observed values. It is suggested that although mathematical functions of dielectric constant-distance may be selected on the basis of agreement between calculation and experiment and of convenience of use, not too much physical significance should necessarily be attributed to these functions.