DESOLVATION EFFECTS ON THE DISSOCIATION-ENERGY OF DIATOMIC-MOLECULES - AB-INITIO STUDY OF THE DISSOCIATION OF LI-F IN POLAR MEDIA

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 electrostat ic solute-solvent interaction is accounted for by the generalized Born formalism introduced through a perturbation approach. The calculation s 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 molecu le 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 angstrom distance. As the two ions come together, a desol vation barrier of about 30 kcal/mol is to be overcome before the forma tion of the neutral Li-F at 1.56 angstrom. The barrier to ionization t owards the ion pair is calculated to be about 14 kcal/mol. The dissoci ation of the ion pair towards the free ions is discussed in terms of t he electrostatic solvation entropy changes.