Ab initio MO and TST calculations for the rate constant of the HNO + NO₂ → HONO + NO reaction

Potential-energy surfaces for various channels of the HNO + NO₂ reaction have been studied at the G2M(RCC,MP2) level, The calculations show that direct hydrogen abstraction leading to the NO + cis-HONO products should be the most significant reaction mechanism. Based on TST calculations of the rate constant, this channel is predicted to have an activation energy of 6-7 kcal/mol and an A factor of ca. 10^-11 cm³ molecule^-1 s^-1 at ambient temperature, Direct H-abstraction giving NO + trans-HONO has a high barrier on PES and the formation of trans-HONO would rather occur by the addition/1,3-H shift mechanism via the HN(O)NO₂ intermediate or by the secondary isomerization of cis-HONO. The formation of NO + HNO₂ can take place by direct hydrogen transfer with the barrier of ca, 3 kcal/mol higher than that for the NO + cis-HONO channel. The formation of HNO₂ by oxygen abstraction is predicted to be the least significant reaction channel. The rate constant calculated in the temperature range 300-5000 K for the lowest energy path producing NO + cis-HONO gave rise to k_a = 7.34 · 10^-20T^{2 64}exp(-2034/T) cm³ molecule^-1 S^-1.