Barrier heights for hydrogen atom transfer reactions. Evaluation of ab initio molecular orbital methods for the degenerate exchange HO• + H2O → H2O + •OH

Several different ab initio molecular electronic structure techniques are used to calculate the barrier height for the reaction HO^• + H₂O → H₂O + ^•OH, which is a prototype for hydrogen abstraction processes involving open-shell radicals and the breaking and formation of strong covalent bonds. The objective of the study is to determine what practical methods are currently available for performing such calculations and to assess their reliability. Methods studied include self-consistent field (ROHF and UHF), multiconfiguration self-consistent field (MCSCF), Møller-Plesset perturbation calculations (up to full fourth order), configuration interaction (CISD), and quadratic configuration interaction (QCISD(T)). It is concluded that, for the calculation of transition-state properties of the important class of reactions studied here, electron correlation, size consistency, and basis set effects are all very important in reducing the computed potential energy barrier from the unrealistically large values obtained with modest levels of theory to the rather small values calculated using more expensive and sophisticated methods. The best estimate for ΔH^‡ is 44 kJ mol^-1 at the basis set superposition error (BSSE) corrected, correlation energy scaled QCISD(T)/6-31++G(3d,2p) level. Inclusion of a tunneling correction of about -16 kJ mol^-1 would further reduce this value in the direction of the estimated experimental barrier height of ΔH^‡ = 22 ± 4 kJ mol^-1.