Excess Electrons in Polar Solvents

Abstract

In this paper we consider a structural model for localized excess electron states in polar solvents with particular reference to dilute metal ammonia solutions and to the hydrated electron. The over-all energetic stability of these species was assessed by considering simultaneously the electronic energy and the medium rearrangement energy. The present model consists of a finite number of loosely packed molecules on the surface of the cavity which are subjected to thermal fluctuations and a polarizable continuum beyond. The electronic energy was computed utilizing an electrostatic microscopic short-range attraction potential, a Landau-type potential for long-range interactions, and a Wigner-Seitz potential for short-range repulsive interactions. The medium rearrangement energy includes the surface tension work, the dipole–dipole repulsion in the first solvation layer, and most importantly the short-range repulsive interactions between the hydrogen atoms of the molecules oriented by the enclosed charge. The gross features of localized electron states in different solvents can be rationalized in terms of different contributions to the medium rearrangement terms. The energetic stability of the localized state of excess electrons in polar solvents was established and the cavity size in the ground state of the solvated electron could be uniquely determined. Experimental energetic and structural data such as volume expansion, coordination numbers, heats of solution, and spectroscopic properties are in qualitative agreement with the predictions of the present model. Optical line shape data calculated from the theoretical model do not agree with experiment; this discrepancy suggests that more data are required in regard to the excited states of the solvated electron.