THE DETERMINANT ROLE OF WATER IN THE IONIC DISSOCIATION OF HO2

The energetics of the ionic dissociation of the hydroperoxyl radical ( HO2) into the H+ and O-2(-) ions in solution is studied using ab initi o methods and water clusters to mimic the solvent effects. The cluster s selected are HO2 ...(H2O)(n), H+...(H2O)(l) and O-2(-)...(H2O)(m), u sing various combinations of n, l and m designed to saturate the first solvation shell of the involved systems, the largest of which contain s 10 water molecules (n = 10) plus the electrostatic field created by the outer solvation shell to these 10 water molecules. We have optimiz ed the geometries of these clusters at the Hartree-Fock level using th e 6-31 + + G(d,p) basis set. The impact of the electronic correlation, using the same basis set, was included using the second-order Moller- Plesset method and the Becke-Lee-Yang-Parr density functional, obtaini ng similar results from both methods. Our results indicate that in ord er to obtain reasonable values of the ionic dissociation energy, one h as to use large clusters which saturate the first solvation shell of t he HO2 molecule and that of the ionic products in an even way. The str uctures of the clusters are given and rationalized.