Arathala Parandaman, Katz Mark, Musah Rabi A
University at Albany-State University of New York, Department of Chemistry, 1400 Washington Avenue, Albany, NY 12222, USA.
Phys Chem Chem Phys. 2020 May 13;22(18):10027-10042. doi: 10.1039/d0cp00570c.
The reactions of thioformaldehyde (H2CS) with OH radicals and assisted by a single water molecule have been investigated using high level ab initio quantum chemistry calculations. The H2CS + ˙OH reaction can in principle proceed through: (1) abstraction, and (2) addition pathways. The barrier height for the addition reaction in the absence of a catalyst was found to be -0.8 kcal mol-1, relative to the separated reactants, which has a ∼1.0 kcal mol-1 lower barrier than the abstraction channel. The H2CS + ˙OH reaction assisted by a single water molecule reduces the barrier heights significantly for both the addition and abstraction channels, to -5.5 and -6.7 kcal mol-1 respectively, compared to the un-catalyzed H2CS + ˙OH reaction. These values suggest that water lowers the barriers by ∼6.0 kcal mol-1 for both reaction paths. The rate constants for the H2CSH2O + ˙OH and OHH2O + H2CS bimolecular reaction channels were calculated using Canonical Variational Transition state theory (CVT) in conjunction with the Small Curvature Tunneling (SCT) method over the atmospherically relevant temperatures between 200 and 400 K. Rate constants for the H2CS + ˙OH reaction paths for comparison with the H2CS + ˙OH + H2O reaction in the same temperature range were also computed. The results suggest that the rate of the H2CS + ˙OH + H2O reaction is slower than that of the H2CS + ˙OH reaction by ∼1-4 orders of magnitude in the temperatures between 200 and 400 K. For example, at 300 K, the rates of the H2CS + ˙OH + H2O and H2CS + ˙OH reactions were found to be 2.2 × 10-8 s-1 and 6.4 × 10-6 s-1, respectively, calculated using [OH] = 1.0 × 106 molecules cm-3, and [H2O] = 8.2 × 1017 molecules cm-3 (300 K, RH 100%) atmospheric conditions. Electronic structure calculations on the H2C(OH)S˙ product in the presence of 3O2 were also performed. The results show that H2CS is removed from the atmosphere primarily by reacting with ˙OH and O2 to form thioformic acid, HO2, formaldehyde, and SO2 as the main end products.
利用高水平从头算量子化学计算方法，研究了硫甲醛（H₂CS）与羟基自由基（˙OH）在单个水分子辅助下的反应。H₂CS + ˙OH反应原则上可以通过以下两种途径进行：（1）氢提取，以及（2）加成途径。发现无催化剂时加成反应的势垒高度相对于分离的反应物为 -0.8 kcal mol⁻¹，比氢提取通道的势垒低约1.0 kcal mol⁻¹。单个水分子辅助的H₂CS + ˙OH反应显著降低了加成和氢提取通道的势垒高度，与未催化的H₂CS + ˙OH反应相比，分别降至 -5.5和 -6.7 kcal mol⁻¹。这些值表明水使两条反应路径的势垒降低了约6.0 kcal mol⁻¹。在200至400 K的大气相关温度范围内，使用正则变分过渡态理论（CVT）结合小曲率隧道效应（SCT）方法计算了H₂CSH₂O + ˙OH和OHH₂O + H₂CS双分子反应通道的速率常数。还计算了相同温度范围内与H₂CS + ˙OH + H₂O反应进行比较的H₂CS + ˙OH反应路径的速率常数。结果表明，在200至400 K的温度范围内，H₂CS + ˙OH + H₂O反应的速率比H₂CS + ˙OH反应的速率慢约1至4个数量级。例如，在300 K时，在[OH] = 1.0 × 10⁶ 分子 cm⁻³和[H₂O] = 8.2 × 10¹⁷ 分子 cm⁻³（300 K，相对湿度100%）的大气条件下，计算得出H₂CS + ˙OH + H₂O和H₂CS + ˙OH反应的速率分别为2.2 × 10⁻⁸ s⁻¹和6.4 × 10⁻⁶ s⁻¹。还对3O₂存在下的H₂C(OH)S˙产物进行了电子结构计算。结果表明，H₂CS主要通过与˙OH和O₂反应从大气中去除，形成硫代甲酸、HO₂、甲醛和SO₂作为主要最终产物。
