Low-temperature curvature in Arrhenius plots of H-atom transfer reactions is usually attributed to tunneling. This paper investigates the feasibility of an alternative mechanism involving nonadiabatic transitions to enhance the low-T rate of O(3P)+H2→OH+H. 3A″ and 1A′ potential surfaces are calculated for the configuration OHH over a range of bond angles. The ab initio surface calculations use two basis sets: double-zeta + polarization (DZP), and DZP augmented by bond functions and Rydberg orbitals (BF), followed by extensive configuration interaction. The CI data points are fitted by a rotated Morse curve-cubic spline functional form. The surface crossing is analyzed in detail. The singlet surface crosses the triplet surface near 120° at an energy which is slightly above (2 kcal/mol) the energy of the lowest triplet barrier (13.2 kcal/mol for the linear geometry). The implications for low-temperature kinetics are discussed.

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Journal The Journal of Chemical Physics
Donaldson, D.J., & Wright, J.S. (1984). Singlet-triplet surface crossings and low-temperature rate enhancement for O(3P)+H2→OH+H. The Journal of Chemical Physics, 80(1), 221–231.