Gas phase reactions of HONO with NO₂, O₃, and HCl: Ab initio and TST study

Gas-phase reactions of HONO with species of atmospheric interest, NO₂, O₃, and HCl, have been investigated by means of ab initio molecular-orbital and transition-state theory calculations. For the HONO+NO₂ reaction, the most favorable pathway is found to be the abstraction of OH in HONO by NO₂, leading to the formation of HNO₃ and NO products. The activation energies computed at the G2M (RCC,MP2) level of theory for the abstraction from trans- and cis-HONO are predicted to be 31.6 and 33.4 kcal/mol, respectively. At 300 K, the calculated rate constant is smaller than 1.2×10^-14 cm³ mol^-1 s^-1. For the HONO+O₃ reaction, the barrier height for the production of HNO₃+O₂ is predicted to be 15.0 kcal/mol with the rate constant at 300 K, 5.7×10^-2 cm³ mol^-1 s^-1. For the trans- and cis-HONO+HCl→H₂O+CINO reactions our G2M calculations predict the corresponding barrier heights of 12.9 and 14.4 kcal/mol with predicted rate constants, 1.3×10² and 0.13×10² cm³ mol^-1 s^-1 at 300 K. The results of our theoretical modeling indicate that the three gas-phase reactions are too slow to be significant under the stratospheric and tropospheric conditions.