Theoretical Estimation of the Activation Energy for the Reaction HO•+ H₂O → H₂O +•OH:  Importance of Tunneling

Ab initio calculations for the potential barrier height for the symmetric H-atom exchange reaction HO • + H 2 O → H 2 O + • OH are reported. A value of 42.2 kJ mol -1 is found using the QCISD(T)/6-311+G(3df,2p) 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 constants calculated by using conventional and small-curvature tunneling-corrected transition state theory with the UMP2/6-311G(d,p) transition structure and reaction path are compared for a wide range of temperatures. Tunneling reduces the Arrhenius activation energy, obtained from the temperature dependence of the calculated rate constants, by at least 20 kJ mol -1 at 300 K. The best theoretical estimate for the Arrhenius activation energy at 300 K is 21.2 kJ mol -1; the discrepancy between this and the experimental value of 17.6 ± 2 kJ mol -1 is likely to be due to neglect of large-curvature tunneling effects. The QCISD(T)/6-311+G(3df,2p) calculated enthalpy of association of HO • + H 2 O → 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 is 25.1 kJ mol -1 .