Reactive quenching of NO (A 2 Σ + ) with H 2 O leads to HONO: a theoretical analysis of the reactive and nonreactive electronic quenching mechanisms.

The electronic quenching of NO (A 2 Σ + ) with molecular partners exemplifies the rich non-adiabatic dynamics that occurs on multiple, coupled potential energy surfaces (PESs). The mechanistic details of the electronic quenching depend sensitively on the nature and strength of the intermolecular interactions between NO (A 2 Σ + ) and the molecular partner. In this paper, we reveal the electronic quenching mechanisms of NO (A 2 Σ + ) with H 2 O, a non-adiabatic process with an extremely large cross section of 121 Å 2 near room temperature. In doing so, we demonstrate that the NO (A 2 Σ + ) + H 2 O PES funnels a wide range of initial intermolecular orientations to the same minimum-energy geometry. Furthermore, we reveal low-energy pathways to conical intersections between NO (A 2 Σ + ) + H 2 O and NO (X 2 Π) + H 2 O that primarily involve decreasing the intermolecular distance and elongating a single O-H bond of H 2 O. Based on these geometric distortions, we predict that nonreactive electronic quenching will be associated with significant vibrational excitation in a local O-H stretch mode in H 2 O. Reactive quenching will produce a H-atom and HONO, an important intermediate in atmospheric and combustion chemistry and a precursor to the hydroxyl radical. Overall, our work provides the first detailed theoretical study of the mechanism of the electronic quenching of NO (A 2 Σ + ) with a polyatomic molecular partner, as well as makes concrete predictions to inform future velocity map imaging experiments.