Theoretical studies of intersystem crossing effects in the O(3P,1D)+H2 reaction

We have studied the influence of intersystem crossing on the reaction dynamics of the O+H-2 reaction by performing trajectory surface hopping (TSH) calculations with accurate potential-energy surfaces and global spin-orbit coupling surfaces that we have generated using a four state model proposed by Hoffmann and Schatz. In the TSH calculations, we develop a new mixed representation that treats the reactant and product asymptotes in the adiabatic representation, and the singlet-triplet crossing region in the diabatic representation. This representation thus correctly describes O and OH fine structure-resolved cross sections, and it also treats intersystem crossing effects arising from the singlet-triplet crossing. Our calculations are based on the 1 (3)A' and 1 (3)A' states of Walch and Kuppermann, and the 1 (1)A' state of Dobbyn and Knowles. The globally determined spin-orbit coupling matrix is derived from complete active space self-consistent field calculations using the two-electron Breit-Pauli Hamiltonian. Our dynamics calculations show that the triplet O+H-2 cross section is modestly increased (up to 20% at collision energies >10 kcal/mol above the reactive threshold) by intersystem crossing, and product rotational excitation is also increased. In addition, we find that the OH spin-orbit distributions favor the (2)Pi(3/2) state by a 2:1 ratio over (2)Pi(1/2). This result is consistent with observations for O atom reactions with alkanes. (C) 2003 American Institute of Physics.