Theoretical studies of intersystem crossing effects in the O+H-2 reaction

We present a general procedure for studying intersystem crossing effects in bimolecular chemical reactions, along with an application of this to the O + H-2 reaction. In this procedure, we use previously derived singlet and t riplet potential energy surfaces that were based on high quality multirefer ence configuration interaction (MRCI) nonrelativistic electronic structure calculations, and the coupling surface is obtained from lower level complet e active space self-consistent field (CASSCF) calculations using the effect ive nuclear charge one-electron Breit-Pauli expression for the spin-orbit i nteraction. We find that the resulting spin-orbit splittings match the know n values for O(P-3), O(D-1), and OH((II)-I-2) sufficiently accurately to be useful for dynamics calculations. Also, the electronic basis can be trunca ted to seven states (1 (3)A', 1 (3)A", and 1 (1)A') without seriously disto rting these asymptotic splittings. We show that the seven states may be exa ctly decoupled into a set of four, which contains the singlet, and a set of three states from the triplets. We find that the spin-orbit matrix element s vary smoothly with geometry, so that a relatively simple function can be used to interpolate matrix elements for all geometries. The cross sections for reaction are calculated using a trajectory surface-hopping (TSH) approa ch in conjunction with a ''diabatic'' representation based on the nonrelati vistic potentials and the CASSCF spin-orbit coupling matrix. An application of this approach is presented to the O + H-2 reaction, using the 1 (1)A' s tate of Dobbyn and Knowles, and 1 (3)A' and 1 (3)A" states of Walch and Kup permann [slightly modified so that they are asymptotically degenerate in th e product (H + OH) region]. The states show a singlet-triplet (S-T) crossin g that is generally on the product side of the barrier on the triplet surfa ces. The TSH results indicate that only a few percent of the trajectories u ndergo intersystem crossing (either from singlet to triplet, or vise versa) at the S-T crossing, so the effect of these transitions on measurable prop erties of the reaction dynamics is small. However, those trajectories that undergo triplet to singlet transition have much higher product rotational e xcitation than those that react on the triplet alone. We find that a much l arger fraction of trajectories (20%-40%) undergo hopping between the two tr iplet states, and this leads to an averaging of the dynamical results for t he two states. (C) 2000 American Institute of Physics. [S0021-9606(00)00745 -5].