Stability of ionic and radical molecular dissociation pathways for reaction in supercritical water

Molecular dynamics simulations have been used to examine the effect of supe rcritical water solvent density on the competition between reaction pathway s for the dissociation step of a model S(N)1 reaction. The effects are inve stigated using an empirical valence bond theory that explicitly includes th e effects of solvation, particularly those on the diabatic ionic state. At low supercritical water densities, the solvent stabilization is insufficien t to give rise to a local minimum on the free energy surface corresponding to a contact ion pair intermediate, although the free energy surface is com pletely ionic in character to solvent densities less than 0.05 g cm(-3). Th e nature of the surface is also changed by solvent density; the change from a mostly covalent (80%) molecule to completely ionic dissociation products is decreasingly rapid as supercritical water density is decreased. Radial density functions reflect how solvation changes along the reaction coordina te and how local density enhancement provides the solvation required to sta bilize the ionic products. These calculations indicate that the diabatic io nic state is lowest in free energy until extremely low supercritical water solvent density (similar to0.03 g cm(-1)), considerably lower than would be expected if local density enhancement were ignored, as in a simple Born mo del calculation. The free energy difference between the two pure states at the dissociation plateau indicates that covalent products may be expected t o reach approximately 22% of the total at the lowest density (0.0435 g cm(- 3)) considered here.