Effect of roaming transition states upon product branching in the thermal decomposition of CH₃NO₂

The kinetics for the thermal unimolecular decomposition of CH ₃NO₂ and its structural isomer CH₃ONO have been investigated by statistical theory calculations based on the potential energy surface calculated at the UCCSD(T)/CBS and CASPT3(8, 8)/6-311+G(3df,2p) levels. Our results show that for the decomposition of CH₃NO₂ at pressures less than 2 Torr, isomerization to CH₃ONO via the recently located roaming transition state is dominant in the entire temperature range studied, 400-3000 K. However, at higher pressures, the formation of the commonly assumed products, CH₃ + NO₂, becomes competitive and at pressures higher than 200 Torr the production of CH₃ + NO₂ is exclusive. The predicted rate constants for 760 Torr and the high-pressure limit with Ar as diluent in the temperature range 500-3000 K, producing solely CH₃ + NO₂, can be expressed respectively by k _d⁷⁶⁰(CH₃NO₂) = 2.94 × 10 ⁵⁵T^-12.6 exp(-35500/T) s^-1 and k _d^∞(CH₃NO₂) = 5.88 × 10²⁴T^-2.35 exp(-31400/T) s^-1. In the low pressure limit, the decomposition reaction takes place exclusively via the roaming TS producing internally excited CH₃ONO, giving rise to both CH₃O + NO and CH₂O + HNO with the second-order rate constant k_d⁰(CH₃NO₂) = 1.17 × 10³¹T^-10.94 exp(-32400/T) cm³ molecule ^-1 s^-1. For CH₃ONO decomposition, a new roaming transition state connecting to the CH₂O + HNO products has been located, lying 6.8 kcal/mol below the well-known four-member ring tight transition state and 0.7 kcal/mol below CH₃O + NO. The rate constants predicted by similar calculations give rise to the following expressions for the thermal decomposition of CH₃ONO in He: k_d ⁷⁶⁰(CH₃ONO) = 8.75 × 10⁴¹T ^-8.97 exp(-22600/T) s^-1 and k_d ^∞(CH₃ONO) = 1.58 × 10²³T ^-2.18 exp(-21100/T) s^-1 in the temperature range 300-3000 K. These results are in very good agreement with available experimental data obtained under practical pressure conditions. The much different branching ratios for the formation of CH₃O + NO and CH₂O + HNO in the decomposition of both CH₃NO₂ and CH₃ONO are also given in this work.