Theoretical interpretation of the kinetics and mechanisms of the HNO + HNO and HNO + 2NO reactions with a unified model

Previously measured decay rates of HNO in the presence of NO have been kinetically modeled on the basis of thermochemical data calculated with the BAC-MP4 technique. The results of this modeling, aided by TST-RRKM calculations for the association of HNO and the isomerization, decomposition, and stabilization of the many dimers of HNO, reveal that the decay of HNO under NO-lean conditions occurs primarily by association forming cis- and trans-(HNO)₂ at temperatures below 420 K. N₂O, which is a relatively minor product, is believed to be formed by H₂O elimination from cis-HON ? NOH, a product of succesive isomerization reactions: trans-(HNO)₂^? → HN(OH)NO^? → HN(O)NOH^? → cis-HON NOH^?. The calculated rate constants, which fit experimental data quantitatively, can be represented by k = 10^16.2 × T^?2.40e^?590/T cm³/mol sec for the HNO recombination reaction and k = 10^?2.44T^3.98e^?600/T cm³/mol sec for N₂O formation in the temperature range 80–420 K, at a total pressure of 710 torr H₂ or He. Under NO-rich conditions, HNO reacts predominantly by the exothermic termolecular reaction, HNO + 2NO → HN(NO)ONO → HN NO + NO₂, with a rate contant of (6 ± 1) × 10⁹ cm⁶/mol² sec at room temperature, based on both HNO decay and NO₂ production. All existing thermal kinetic data on HNO + HNO and HNO + 2NO processes can be satisfactorily rationalized with a unified model based on the thermochemical data obtained by BAC-MP4 calculations.