Mechanism of hydroxyl radical generation from a silica surface: Molecular orbital calculations

The interaction of an H2O molecule with cluster models of fractured silica surfaces was studied by means of quantum mechanical calculations. Two clusters representing homolytic cleavage (equivalent to Si-center dot and equivalent to SiO center dot) and two representing heterolytic cleavage (equivalent to Si+ and equivalent to Si-O-) of silica surfaces were modeled. Vibrational frequencies of the reactants and products of these silica surfaces reacting with H2O have been calculated and compare favorably with experiment. Comparisons of the Gibbs free and potential energies for the model ionic and radical states were made, and the radical pair of sites was predicted to be more stable by approximately -70 to -85 kJ/mol, depending on the computational methodology. These calculations suggest that when silica is fractured in a vacuum homolytic cleavage is favored. Reaction pathways were investigated for these four model surface sites interacting with H2O. The reaction of H2O with equivalent to SiO center dot was predicted to generate OH center dot. Rate constants for these reactions were also calculated and predict a rapid equibrium for the reaction equivalent to SiO center dot + H2O -> equivalent to SiOH + OH center dot.. Stability of a finite number of equivalent to SiO center dot sites at equilibrium in the above reaction with H2O was also predicted, which implies a long-term ability of silica surfaces to produce OH center dot radicals if the sites of the broken bonds do not repolymerize to form siloxane groups.