Study of electronic structure and dynamics of interacting free radicals influenced by water.

We present a study of electronic structure, stability, and dynamics of interaction and recombination of free radicals such as HO(2) and OH influenced by water. As simple model calculations, we performed ab initio and density functional calculations for the interaction of HO(2) and OH in the presence of water cluster. Results indicate that a significant interaction, overcoming the repulsive Columbic barrier, occurs at a separation distance between the radicals of 5.7 A. This confirms early predictions of the minimum size of molecular dianions stable in the gas phase. It is well known that atomic dianions are unstable in the gas phase but molecular dianions are stable when the size of the molecule is larger than 5.7 A. Ab initio molecular dynamics calculations with Car-Parrinello scheme show that the reaction is very fast and occurs on a time scale of about 1.5 ps. The difference in stability and dynamics of the interacting free radicals on singlet and triplet potential energy surfaces is also discussed.