Overtone-induced reactions on the HO2NO2 potential surface

We report density-functional theory calculations performed at the B3LYP/6-311++G(3df,3pd) level on the lowest energy singlet and triplet potential energy surfaces of the HO2NO2 System. The reaction forming HONO + ground-state oxygen, O-2 ((3)Sigma), from HO2 + NO2 proceeds via a triplet transition state which lies about 2 kcal/mol above reagents, and about 22 kcal/mol above ground-state HO2NO2. Dissociation of ground-state pernitric acid into HO2 + NO2 is energetically accessible through excitation of the nu = 2 overtone level of the OH stretching vibration (v(OH) = 2). Overtone excitation into VOH L 3, followed by a singlet-triplet potential surface crossing, could lead to HONO + O-2 ((3)Sigma) products. On the singlet potential surface, the energetic barrier to the reaction forming HONO + O-2 ((1)Sigma) lies very near the product energy. This product channel may be accessible via excitation of the v(OH) = 3 or 4 overtones. Complexation with a single water molecule does not alter the barrier height to reaction, but may promote reaction by changing the strength and or location of the singlet-triplet coupling. The reaction of HONO (v(OH) greater than or equal to 3) with O-2 ((3)Sigma) may form HO2 + NO2 in polluted urban atmospheres with a rate several orders of magnitude smaller than that of HONO photolysis.