Importance of intersystem crossing in the S( ³P, ¹D) + H ₂ → SH + H reaction

A "mixed" representation approach in conjunction with a trajectory surface-hopping method is used to study intersystem crossing effects in the S + H ₂ reaction. These calculations are based on high-quality potential surfaces that we have determined for the two lowest triplet states of SH ₂ and globally determined spin-orbit coupling matrix elements that are obtained from CASSCF calculations. A previously determined surface for the lowest singlet state (Ho, T.-S.; Hollebeek, T.; Rabitz, H.; Chao, S. D.; Skodje, R. T.; Zyubin, A. S.; Mebel, A. M. J. Chem. Phys. 2002, 116, 4124) is also used. We find that in contrast to the O( ³P) + H ₂ reaction, which we studied previously at the same level, there is significant intersystem crossing in the S( ³P) + H ₂ reaction. In particular, for the reaction starting from triplet S + H ₂ close to the threshold, the dominant mechanism involves intersystem crossing to the singlet state prior to encountering the triplet barrier, and as a result, the thermal rate constant at low temperatures is controlled by intersystem crossing. This behavior occurs in part because the spin-orbit coupling is about 3 times larger in S than in O, but another important factor is the location of the singlet/triplet crossing, which occurs on the reagent side of the triplet barrier in S + H ₂ and on the product side in O + H ₂. We also find that trajectories that undergo a triplet-to-singlet transition have higher product rotational excitation than those that remain on the triplet surfaces. For the S( ¹D) + H ₂ reaction, we find significant electronic quenching due to intersystem crossing, leading to a factor of 2 or more reduction in the reactive cross section, and a much flatter dependence of the cross section on collision energy for energies above 2.5 kcal/mol. This result agrees with recent molecular beam measurements.