Ab initio study of the HO2+NO reaction: Prediction of the total rate constant and product branching ratios for the forward and reverse processes

The mechanisms for HO2+NO and its reverse reactions have been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the B3LYP/6-311+G(3df,2p) level and their energies refined by single-point calculations with the highest scheme of the modified Gaussian-2 method. Ab initio results show that formation of HO+NO2 from HO2+NO by the direct fragmentation of the peroxynitrous acid, HOONO intermediate, is predominant; the alternative path occurring by the isomerization of HOONO to HONO2 is 5.2 kcal/mol less favorable. The stepwise formation of HNO+O2 from HOONO is energetically unfavorable; the barriers for the direct H abstract reactions via singlet and triplet paths are found to be rather high also. Rate constant calculations show that the forward reaction is pressure independent below 10 atm; the reverse OH+NO2 reactions producing HONO2 and HOONO appear to be strongly pressure dependent; below 1 atm, the yield of HOONO from HO+NO2 is 2.5% at 300–400 K, it reaches 12.2% and 9.1%, respectively, at 300 and 400 K at 3.8×104 Torr pressure. The low- and high-pressure rate constants with He as a third-body for the formation of HOONO and HONO2 from the HO+NO2 reaction can be expressed by k0 (HOONO)=3.15×102 T−12.3 exp(−585/T), k0 (HONO2)=3.32×10−6 T−8.8 exp(−1569/T)cm6 molecule−2 s−1 and k∞ (HOONO)=1.71×10−10 T−0.24 exp(100/T) and k∞ (HONO2)=4.74×10−9 T−0.82 exp(21/T) cm3 molecule−1 s−1, respectively, in the temperature range of 200–2000 K. The unimolecular decomposition rate constant (in Ar) of HNO3 can be expressed as kd∞ (HNO3)=2.30×1023 T−2.27 exp(−26317/T) s−1 and kd0 (HNO3)=1.27×1015 T−6.55exp(−26038/T) cm3 molecule−1 s−1, respectively. The predicted values are all in close agreement with experimental data for both forward and reverse processes.