THEORETICAL ESTIMATION OF THE ACTIVATION-ENERGY FOR THE REACTION HO-CENTER-DOT-]H2O+CENTER-DOT-OH - IMPORTANCE OF TUNNELING(H2O)

Ab initio calculations for the potential barrier height for the symmet ric H-atom exchange reaction HO. + H2O --> H2O + (OH)-O-. are reported . A value of 42.2 kJ mol(-1) is found using the QCISD(T)/6-311+G(3df,2 p) method. Multireference CISD calculations converge toward a similar value for the barrier provided that a Davidson correction is applied. The effect of quantum mechanical tunneling is investigated. Rate const ants calculated by using conventional and small-curvature tunneling-co rrected transition state theory with the UMP2/ 6-311G(d,p) transition structure and reaction path are compared for a wide range of temperatu res. Tunneling reduces the Arrhenius activation energy, obtained from the temperature dependence of the calculated rate constants, by at lea st 20 kJ mol(-1) at 300 K. The best theoretical estimate for the Arrhe nius activation energy at 300 K is 21.2 kJ mol(-1); the discrepancy be tween this and the experimental value of 17.6 +/- 2 kJ mol(-1) is like ly to be due to neglect of large-curvature tunneling effects. The QCIS D(T)/6-311+G(3df,2p) calculated enthalpy of association of HO. + H2O - -> HO.... HOH, the hydrogen-bonded precursor complex, is -8.9 kJ mol(- 1). The best theoretical estimate for the intrinsic barrier height for the symmetric H-atom exchange HO.... HOH --> HOH ...(OH)-O-. is 25.1 kJ mol(-1).