Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
J Phys Chem A. 2013 Aug 15;117(32):7308-13. doi: 10.1021/jp401148q. Epub 2013 Jun 26.
The kinetics for the thermal unimolecular decomposition of CH3NO2 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 kd(760)(CH3NO2) = 2.94 × 10(55)T(-12.6) exp(-35500/T) s(-1) and kd(∞)(CH3NO2) = 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 CH3ONO, giving rise to both CH3O + NO and CH2O + HNO with the second-order rate constant kd(0)(CH3NO2) = 1.17 × 10(31)T(-10.94) exp(-32400/T) cm(3) 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: kd(760)(CH3ONO) = 8.75 × 10(41)T(-8.97) exp(-22600/T) s(-1) and kd(∞)(CH3ONO) = 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 CH3O + NO and CH2O + HNO in the decomposition of both CH3NO2 and CH3ONO are also given in this work.
CH3NO2 和其结构异构体 CH3ONO 的热单分子分解动力学已通过基于 UCCSD(T)/CBS 和 CASPT3(8, 8)/6-311+G(3df,2p) 水平计算的统计理论计算进行了研究。我们的结果表明，对于压力小于 2 托的 CH3NO2 分解，在整个研究温度范围内，通过最近定位的 roaming 过渡态异构化为 CH3ONO 是主要的，温度范围为 400-3000 K。然而，在更高的压力下，通常假设的产物 CH3 + NO2 的形成变得具有竞争力，并且在压力高于 200 托时，CH3 + NO2 的产生是独占的。在温度范围为 500-3000 K 下，以 Ar 作为稀释剂且仅产生 CH3 + NO2 的 760 Torr 和高压极限下的预测速率常数可以分别表示为 kd(760)(CH3NO2) = 2.94 × 10(55)T(-12.6) exp(-35500/T) s(-1) 和 kd(∞)(CH3NO2) = 5.88 × 10(24)T(-2.35) exp(-31400/T) s(-1)。在低压极限下，分解反应仅通过 roaming TS 进行，产生内部激发的 CH3ONO，从而产生 CH3O + NO 和 CH2O + HNO，二级速率常数 kd(0)(CH3NO2) = 1.17 × 10(31)T(-10.94) exp(-32400/T) cm(3) molecule(-1) s(-1)。对于 CH3ONO 分解，已经定位了一个新的 roaming 过渡态，连接到 CH2O + HNO 产物，位于著名的四元环紧过渡态以下 6.8 kcal/mol，低于 CH3O + NO 以下 0.7 kcal/mol。类似计算预测的速率常数给出了以下在 He 中 CH3ONO 热分解的表达式：kd(760)(CH3ONO) = 8.75 × 10(41)T(-8.97) exp(-22600/T) s(-1) 和 kd(∞)(CH3ONO) = 1.58 × 10(23)T(-2.18) exp(-21100/T) s(-1)，温度范围为 300-3000 K。这些结果与在实际压力条件下获得的可用实验数据非常吻合。在 CH3NO2 和 CH3ONO 的分解中，CH3O + NO 和 CH2O + HNO 的形成的分支比也有很大的不同。
