## Abstract

The kinetics for the thermal unimolecular decomposition of CH _{3}NO_{2} and its structural isomer CH_{3}ONO 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 CH_{3}NO_{2} at pressures less than 2 Torr, isomerization to CH_{3}ONO 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, CH_{3} + NO_{2}, becomes competitive and at pressures higher than 200 Torr the production of CH_{3} + NO_{2} 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 CH_{3} + NO_{2}, can be expressed respectively by k _{d}
^{760}(CH_{3}NO_{2}) = 2.94 × 10 ^{55}T^{-12.6} exp(-35500/T) s^{-1} and k _{d}
^{∞}(CH_{3}NO_{2}) = 5.88 × 10^{24}T^{-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 CH_{3}ONO, giving rise to both CH_{3}O + NO and CH_{2}O + HNO with the second-order rate constant k_{d}
^{0}(CH_{3}NO_{2}) = 1.17 × 10^{31}T^{-10.94} exp(-32400/T) cm^{3} molecule ^{-1} s^{-1}. For CH_{3}ONO decomposition, a new roaming transition state connecting to the CH_{2}O + 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 CH_{3}O + NO. The rate constants predicted by similar calculations give rise to the following expressions for the thermal decomposition of CH_{3}ONO in He: k_{d}
^{760}(CH_{3}ONO) = 8.75 × 10^{41}T ^{-8.97} exp(-22600/T) s^{-1} and k_{d}
^{∞}(CH_{3}ONO) = 1.58 × 10^{23}T ^{-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 CH_{3}O + NO and CH_{2}O + HNO in the decomposition of both CH_{3}NO_{2} and CH_{3}ONO are also given in this work.

Original language | English |
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Pages (from-to) | 7308-7313 |

Number of pages | 6 |

Journal | Journal of Physical Chemistry A |

Volume | 117 |

Issue number | 32 |

DOIs | |

State | Published - 15 Aug 2013 |

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