BOND-DISSOCIATION IN HYDROGEN-PEROXIDE AND IN THE HYDROGEN-PEROXIDE RADICAL-ANION - A-MO ABINITIO MCSCF APPROACH

The problem of bond dissociation in hydrogen peroxide and in its monoanion was tackled with the ab initio MCSCF method by employing a triple-zeta (TZP) contracted basis set, augmented by polarization functions. For hydrogen peroxide the calculated energies for homolytic O-O and O-H bond cleavages are underestimated (by about 12 kcal mol-1) with respect to bond dissociation enthalpies obtained from heats of formation. This result seems to arise from not taking dynamical correlation effects properly into account, as emerges from the comparison with the results of MP2/TZP calculations. For the monoanion of hydrogen peroxide the equilibrium geometry turns out to be almost all planar with one hydrogen atom shifted between the oxygen atoms and the unpaired electron mostly localized on the outer oxygen atom. The two processes: H2O2- (X 2A)-->OH(X 2PI) + -OH(X 1SIGMA+) and H2O2-(X 2A)-->O-(X 2p(u)) + H2O(X 1A1) require 36.15 and 23.82 kcal mol-1, respectively (41.06 and 30.14 kcal mol-1 at MP2/TZP level), and the latter is likely to be the more favoured path in the chemical reactivity of hydrogen peroxide in the presence of electron donating molecules (i.e. Fenton's reagent).