Gas-phase reactions of HONO with HNO and NH3: An Ab initio MO/TST study

The reduction of HONO by HNO and NH₃ has been investigated by means of ab initio molecular orbital and transition-state theory (TST) calculations. The main reaction channels for the HNO + trans-HONO (cisHONO) reactions are those proceeding via five-member ring transition states, leading to the production of NO and H₂O. In the temperature range 300-1000 K, TST calculations predict an A factor of 2.25 × 10¹⁰ cm³ mol^-1 s^-1 (or 3.63 × 10¹⁰ cm³ mol^-1 s^-1) and an apparent activation energy of 20.9 kcal/mol (or 21.9 kcal/mol) for the HNO + trans-HONO (or m-HONO) reaction. In the NH₃ + HONO system, the reaction NH₃ + cis/trans-HONO → H₂NNO + H₂O with barrier heights centering around 34 kcal/mol can occur at high temperatures. The reversible H-atom exchange reaction NH₃ + cis-HONO ⇔ H₂NH-O(H)NO ⇔ NH₂H + cis-HONO occurs readily. The calculated rate constant for the reaction at 300 K is 1.06 × 10⁶ cm3 mol^-1 s^-1, in reasonable agreement with the experimental value of 2.2 × 10⁶ cm³ mol^-1 s^-1.