THEORETICAL-STUDIES OF THE HYDROGEN-PEROXIDE POTENTIAL SURFACE .2. AN ABINITIO, LONG-RANGE, OH(2-PI) + OH(2-PI) POTENTIAL

Ab initio, multireference, CI calculations have been used to characterize the long-range interaction potentials between two OH radicals. The calculations predict that the four singlet surfaces correlating with ground-state OH radicals interact very strongly, resulting in a complex addition reaction path. At very long range (O-O separations of more than 8 angstrom) the character of the surfaces is dominated by the electrostatic, dipole-dipole interaction, and consequently, the four surfaces are nearly degenerate. At intermediate distances (O-O separations of 3-6 angstrom) hydrogen bonding is found to be the dominant attractive force. In this region the four singlet surfaces split into two pairs, with one pair being significantly more attractive than the other. At shorter distances (O-O separations less than 2.5 angstrom), covalent bonding becomes important. In this region there is only one attractive singlet surface. Sharp, avoided crossings occur at O-O separations between 2.5 and 3 angstrom where the lowest energy wave function switches from hydrogen bonding in character to covalent bonding. In this region, the orientation of the minimum-energy path also changes rapidly from a hydrogen-bonding orientation (OOH angle of 0-degrees) to a covalent bonding orientation (OOH angle of 90-degrees).