The role of the C2v(1 2B2–1 2A1),Cs(1 2A–2 2A), and C∞v(1 2Π–1 2Σ+) seams of conical intersection in the dynamics of the nonadiabatic quenching of OH(A 2Σ+) by molecular hydrogen is investigated. The locus of the C2v and C∞v conical intersections and the Cs seam bridging them is determined. The accessibility of these seams following optical excitation from the ground 1 2B2 state of the OH–H2 van der Waals complex is examined. Using linear interpolation pathways on the 2 2A potential-energy surface connecting these seams and the Franck–Condon region for vertical excitation as a guide, a transition state separating the 2 2A van der Waals complex from the seam of conical intersection was located. The barrier of 8.44 kcal mol−1 is due primarily to the required rotation of the OH molecule relative to H2. Finally, gradient descent paths from the conical intersection were determined to identify the possible products of the nonadiabatic quenching. For each seam, pathways leading to both OH+H2 and H2O+H products were found.

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