On the mechanism for the nonadiabatic reactive quenching of OH(A2Σ+) by H2(1$\Sigma _{\rm g}^+$Σg+): The role of the 22A state

Abstract

A scheme for reactive electronic quenching of OH(A(2)Σ(+)) through collisions with H2 is proposed, supported by electronic structure data obtained from multireference configuration interaction wave functions. The scheme represents an insertion pathway that leads from the initial 3(2)A state in the reactant channel, into a valence region, where a nonadiabatic transition to the 2(2)A state, enabled by a 2(2)A-3(2)A conical intersection seam occurs. Once on the 2(2)A state, insertion of HO into H2 provides access to a linking region and, after surmounting a small barrier, to a region where the low-lying electronic states are Rydberg in character, corresponding to the 3s, 3p(x), 3p(y), and 3p(z) states of OH3(+). In the Rydberg region, a deep well on the 2(2)A potential energy surface exists. Direct passage from the 2(2)A state to ground state products, H2O(X(1)A1) + H, is precluded by an energy barrier so that an intermediate complex can be formed on the 2(2)A potential energy surface. As the insertion is facilitated by rehybridization of the oxygen orbitals from sp to sp(3) in the linking region, nonplanar approach of HO to H2 is favored. The precipitous change in electronic structure from valence to Rydberg character renders the linking region inaccessible on the 3(2)A potential energy surface. From the 2(2)A state in the Rydberg region, access to the H2O + H product channel is enabled by repeated passage through a region of appreciable 1(2)A-2(2)A derivative coupling or by radiative decay. This scheme supplements other pathways in which nonadiabatic transitions from the 2(2)A state to the 1(2)A state in the valence region enable both planar and nonplanar insertion and abstraction paths leading directly to H2O products.