Seams of Conical Intersections Relevant to the Quenching of OH(A²Σ⁺) by Collisions with H₂

Using multireference configuration interaction wave functions composed of 17–52 million configuration state functions, 18 points on the 1²A–2²A seam and 162 points on the 2²A–3²A seam of conical intersections relevant to the collisional quenching of OH­(A²Σ⁺) by H₂ are determined and analyzed. In the vicinity of planar nuclear configurations, the former seam corresponds to a 1²A′–1²A″ seam of intersection and the latter corresponds to a 1²A′–2²A′ seam. For the previously studied 2²A–3²A seam, two regions not previously examined are reported: (i) an out-of-plane region that connects smoothly to the 1²A′–2²A′ seam for planar structures and (ii) a Rydberg region that includes D_3h/C_3v structures where the 2²A–3²A seam is a 2²A′–3²A′ seam for D_3h structures. Some of the nonplanar points on the 2²A–3²A seam of conical intersection are found to have OH and H₂ distances comparable to those of the reactant molecules and energies below that of the reactant asymptote. These nonplanar entrance channel conical intersections suggest new mechanisms for the quenching reaction. The Rydberg region introduces new connectivity and symmetry issues. For the 1²A–2²A [1²A′–1²A″] seam, which unlike the 2²A–3²A [1²A′–2²A′] seam cannot continuously deform from planar to nonplanar structures except through confluences, no evidence of nonplanar points on the conical intersection seam was found. The continuous conical parameters, g^I,J, h^I,J, s_x^I,J, and s_y^I,J and the associated vectors g^I,J, h^I,J, and s^I,J, are determined and discussed. The conical parameters are made continuous by a prescribed rotation of the degenerate wave functions. The continuity of these conical parameters makes it possible to construct a quasi-diabatic representation of the coupled adiabatic potential energy surfaces.