Chen Ting-Yu, Lee Yuan-Pern
Department of Applied Chemistry and Institute of Molecular Science National Chiao Tung University, Hsinchu 30010, Taiwan.
Phys Chem Chem Phys. 2020 Aug 18;22(31):17540-17553. doi: 10.1039/d0cp01940b.
The reaction CH2I + O2 has been widely employed recently for the production of the simplest Criegee intermediate CH2OO in laboratories, but the detailed dynamics of this reaction have been little explored. Infrared emission of several products of this reaction, initiated on irradiation of CH2I2 and O2 (∼8 Torr) in a flowing mixture at 308 or 248 nm, was recorded with a step-scan Fourier-transform spectrometer; possible routes of formation were identified according to the observed vibrational distribution of products and published theoretical potential-energy schemes. Upon irradiation at 308 nm, Boltzmann distributions of CO (v ≤ 5, J ≤ 19) with an average vibrational energy of 32 ± 3 kJ mol-1 and OH (v ≤ 3, J ≤ 5.5) with an average vibrational energy of 29 ± 4 kJ mol-1 were observed and assigned to the decomposition of HCOOH* to form CO + H2O and OH + HCO, respectively. The broadband emission of CO2 was simulated with two vibrational distributions of average energies (91 ± 4) and (147 ± 8) kJ mol-1 and assigned to be produced from the decomposition of HCOOH* and methylene bis(oxy), respectively. Upon irradiation of samples at 248 nm, the emission of OH and CO2 showed similar distributions with slightly greater energies, but the distribution of CO (v ≤ 11, J ≤ 19) became bimodal with average vibrational energies of (23 ± 4) and (107 ± 29) kJ mol-1, and branching (56 ± 5) : (44 ± 5). The additional large-v component is assigned to be produced from a secondary reaction HCO + O2 to form CO + HO2; HCO is a coproduct of OH. The branching between CO and OH is (50 ± 5) : (50 ± 5) at 308 nm and (64 ± 5) : (36 ± 4) at 248 nm, consistent with the mechanism according to which an additional channel to produce CO opens at 248 nm. Highly internally excited H2CO was also observed. With O2 at 16 Torr, the extrapolated nascent internal distributions are similar to those with O2 at 8 Torr except for a slight quenching effect.
反应CH₂I + O₂最近在实验室中被广泛用于制备最简单的克里吉中间体CH₂OO，但该反应的详细动力学鲜有研究。在308或248 nm下，对流动混合气体中CH₂I₂和O₂（约8 Torr）进行辐照引发此反应，用步进扫描傅里叶变换光谱仪记录了几种产物的红外发射；根据观察到的产物振动分布和已发表的理论势能图确定了可能的形成途径。在308 nm辐照时，观察到CO（v≤5，J≤19）的玻尔兹曼分布，平均振动能为32±3 kJ mol⁻¹，OH（v≤3，J≤5.5）的玻尔兹曼分布，平均振动能为29±4 kJ mol⁻¹，分别归因于HCOOH分解形成CO + H₂O和OH + HCO。用平均能量为(91±4)和(147±8) kJ mol⁻¹的两种振动分布模拟了CO₂的宽带发射，分别归因于HCOOH和亚甲基双(氧基)的分解。在248 nm辐照样品时，OH和CO₂的发射显示出相似的分布，能量略高，但CO（v≤11，J≤19）的分布变为双峰，平均振动能为(23±4)和(107±29) kJ mol⁻¹，分支比为(56±5) : (44±5)。额外的大v分量归因于二级反应HCO + O₂形成CO + HO₂；HCO是OH的副产物。在308 nm时CO和OH之间的分支比为(50±5) : (50±5)，在248 nm时为(64±5) : (36±4)，这与在248 nm时打开了一个额外的生成CO通道的机理一致。还观察到了高度内部激发的H₂CO。当O₂为16 Torr时，除了轻微的猝灭效应外，推断的初始内部分布与O₂为8 Torr时相似。
