An Experimental and Master Equation Investigation of Kinetics of the CH2OO + RCN Reactions (R = H, CH3, C2H5) and Their Atmospheric Relevance

We have performed direct kinetic measurements of the
CH2OO + RCN reactions (R = H, CH3, C2H5) in the temperature range 233–360 K and pressure
range 10–250
Torr using time-resolved UV-absorption spectroscopy. We have utilized
a new photolytic precursor, chloroiodomethane (CH2ICl),
whose photolysis at 193 nm in the presence of O2 produces
CH2OO. Observed bimolecular rate coefficients for CH2OO + HCN, CH2OO + CH3CN, and CH2OO + C2H5CN reactions at 296 K are (2.22
± 0.65) × 10–14 cm3 molecule–1 s–1, (1.02 ± 0.10) ×
10–14 cm3 molecule–1 s–1, and (2.55 ± 0.13) × 10–14 cm3 molecule–1 s–1, respectively, suggesting that reaction with CH2OO is
a potential atmospheric degradation pathway for nitriles. All the
reactions have negligible temperature and pressure dependence in the
studied regions. Quantum chemical calculations (ωB97X-D/aug-cc-pVTZ
optimization with CCSD(T)-F12a/VDZ-F12 electronic energy correction)
of the CH2OO + RCN reactions indicate that the barrierless
lowest-energy reaction path leads to a ring closure, resulting in
the formation of a 1,2,4-dioxazole compound. Master equation modeling
results suggest that following the ring closure, chemical activation
in the case of CH2OO + HCN and CH2OO + CH3CN reactions leads to a rapid decomposition of 1,2,4-dioxazole
into a CH2O + RNCO pair, or by a rearrangement, into a
formyl amide (RC(O)NHC(O)H), followed by decomposition into CO and
an imidic acid (RC(NH)OH). The 1,2,4-dioxazole, the CH2O + RNCO pair, and the CO + RC(NH)OH pair are atmospherically significant
end products to varying degrees.

本研究通过时间分辨紫外吸收光谱技术，在233至360开尔文温度范围和10至250托尔压力范围内，对CH2OO与RCN反应（R分别为H、CH3、C2H5）进行了直接的动力学测量。本研究采用了一种新的光化学前体，即氯碘甲烷（CH2ICl），其在193纳米光解并存在氧气的情况下生成CH2OO。在296开尔文温度下，观察到的CH2OO与HCN、CH2OO与CH3CN以及CH2OO与C2H5CN反应的双分子速率常数分别为（2.22 ± 0.65）× 10–14 立方厘米分子–1秒–1、（1.02 ± 0.10）× 10–14 立方厘米分子–1秒–1和（2.55 ± 0.13）× 10–14 立方厘米分子–1秒–1，这表明与CH2OO的反应可能是腈类物质在大气中降解的一条潜在途径。所有反应在研究区域内均表现出可忽略的温度和压力依赖性。CH2OO与RCN反应的量子化学计算（采用ωB97X-D/aug-cc-pVTZ优化，并辅以CCSD(T)-F12a/VDZ-F12电子能量校正）表明，无能垒的最低能量反应路径导致环闭，从而形成1,2,4-二氧杂唑化合物。主方程模型结果表明，在环闭之后，CH2OO与HCN和CH2OO与CH3CN反应的化学激活会导致1,2,4-二氧杂唑迅速分解成CH2O与RNCO对，或者通过重排形成甲酰胺（RC(O)NHC(O)H），随后分解为CO和亚氨基酸（RC(NH)OH）。1,2,4-二氧杂唑、CH2O与RNCO对以及CO与RC(NH)OH对均为大气中具有不同程度重要性的最终产物。