Theor Chim Acta (1993) 86:211-217 Theorefica Chimica Acta © Springer-Verlag 1993 Desolvation effects on the dissociation energy of diatomic molecules: Ab initio study of the dissociation of Li-F in polar media* J. Lahsen 1, A. Toro-Labbe 1, R. Contreras 1, and A. Aizman 2 1 Departamento de Quimica, Facultad de Ciencias, Universidad de Chile, Casilla 653-Santiago, Chile 2 Departamento de Quimica, Fac. Ciencia, U. F Santa Maria Casilla 110-V, Valparaiso, Chile Received November 1, 1991/Accepted November 9, 1992 Summary. The potential curve of the ground state dissociation of Li-F in water has been studied by a combination of a standard ab initio Hartree-Fock procedure and a perturbative reaction field approach. The electrostatic solute- solvent interaction is accounted for by the generalized Born formalism intro- duced through a perturbation approach. The calculations were carried out at a 6-311 + G* basis set level. Diffuse functions of s symmetry were included to model a desolvation potential. A double well potential curve was obtained for the dissociation of this molecule in the presence of a highly polarizable medium. The first minimum, corresponding to an ion pair, electrostatically bound, is found at a R(Li-F) <6.0/~ distance. As the two ions come together, a desolva- tion barrier of about 30 kcal/mol is to be overcome before the formation of the neutral Li-F at 1.56&. The barrier to ionization towards the ion pair is calculated to be about 14 kcal/mol. The dissociation of the ion pair towards the free ions is discussed in terms of the electrostatic solvation entropy changes. Key words: Dissociation of Li-F in water - Solvation and desolvation effects- Ab initio potential curve of Li-F 1. Introduction Solvent effect is an important factor in chemical reactivity. In many cases solute-solvent interactions lead to changes in electronic and molecular properties that strongly modify the reactivity pattern of a given substrate, with respect to the gas phase process. Many attempts to evaluate such medium effects have been made in the last decades [1-3]. Most of them are based on the Onsager reaction field theory [4] and usually implemented within semiempirical methods of calculation [5-7]. Most reliable attempts to evaluate medium effects within microscopic models have been reported by Warshel and coworkers [8]. We are interested in performing a type of calculation that, retaining the simplicity of the reaction field theory, be able to overcome the problem of describing the gas * Contribution No 6 from Centro de Mecfinica Cuantica Aplicada (CMCA)