Conical intersections and the nonadiabatic reactions $H2O+O(3P)↔OH(A 2Σ+)+OH(X 2Π)$

In this work the reaction $O(3P)+H2O→OH(X 2Π)+OH(A 2Σ+)$ and the reverse reaction were studied. $Π3–3Δ$ and $Π3–3Σ+$ conical intersection seams were found that tie two reactant-channel potential energy surfaces to the two excited state potential energy surfaces correlating with $OH(X 2Π)+OH(A 2Σ+)$ products. Near their lowest energy points, which occur for collinear geometries, these accidental symmetry-allowed conical intersection seams have very similar geometries and are separated by <90 cm⁻¹. This close proximity may reflect a three state conical intersection not required by symmetry. The conical intersection seams were shown to be accessible from both the reactant and product channels without a barrier, above any reaction endoergicity, and provide in the case of the reverse reaction for branching to either $H2O+O(3P)$ or $OH(X 2Π)+OH(X 2Π).$