Kaur Ramanpreet
Quantum Chemistry Group, Department of Chemistry & Centre of Advanced Studies in Chemistry, Panjab University , Chandigarh 160014, India.
J Phys Chem A. 2018 Mar 1;122(8):1926-1937. doi: 10.1021/acs.jpca.7b06916. Epub 2018 Feb 19.
There are some unsettled issues regarding the mechanism and kinetics of an important atmospheric reaction of NO radical with the SH radical. The existing mechanism is based on the formation of HSO and NO radicals, both of which can result only along one barrierless channel. However, the detection of NO radical has never been reported though the formation of HSO radical has been followed in some studies to determine the rate constants. The latter are mainly obtained by monitoring the SH decay, but rate constants are reported to be highly conflicting among the existing studies reporting its value ranging from 10 to 10 cm molecule sec. The present work attempts to resolve these issues by exploring various reaction pathways through the global reaction route mapping of the potential energy surface at the level of spin-unrestricted and spin-restricted coupled-cluster and density functional theories. The initial association of two radicals was found to proceed via two barrierless modes: (1) S-N association leading to HSNO and, (2) S-O association resulting in HSONO, in particular the cis-isomer. The kinetics of the barrierless pathways was investigated through rate constants computed using canonical variational transition state theory (CVTST) along with their temperature and pressure dependence investigated using the master equation. The rate constants calculated using spin-unrestricted methods are found to be in agreement with experimentally observed range of rate constant, and the formation of cis-HSONO (via mode 2) is observed to be the main contributing channel. Contrary to the results of spin-restricted calculations, the barrierless channel (mode 1) leading to the formation of HSNO is predicted to involve two bottlenecks when results using spin-unrestricted calculations were analyzed. Notably, the spin-unrestricted calculations predict a prereaction complex for the formation of S-N bond (via mode 1) which has been treated using Miller's unified transition state theory with a two transition state model. The fate of all the species involved in the reaction is critically evaluated in the present work, and the predictions made can be a subject of further experimental and theoretical studies involving radical-radical reactions.
关于NO自由基与SH自由基这一重要大气反应的机理和动力学，存在一些尚未解决的问题。现有的机理是基于HSO和NO自由基的形成，而这两者都只能沿着一个无势垒通道产生。然而，尽管在一些研究中通过跟踪HSO自由基的形成来确定速率常数，但从未有过NO自由基被检测到的报道。后者主要通过监测SH的衰减来获得，但现有研究报道的速率常数差异很大，其值在10到10 cm³ molecule⁻¹ sec⁻¹之间。本工作试图通过在自旋非限制和自旋限制耦合簇以及密度泛函理论水平上对势能面进行全局反应路线映射来探索各种反应途径，从而解决这些问题。发现两个自由基的初始缔合通过两种无势垒模式进行：(1) S-N缔合生成HSNO，以及(2) S-O缔合生成HSONO，特别是顺式异构体。通过使用正则变分过渡态理论（CVTST）计算速率常数，并使用主方程研究其温度和压力依赖性，对无势垒途径的动力学进行了研究。发现使用自旋非限制方法计算的速率常数与实验观察到的速率常数范围一致，并且观察到顺式HSONO（通过模式2）的形成是主要贡献通道。与自旋限制计算结果相反，当分析自旋非限制计算结果时，预测导致HSNO形成的无势垒通道（模式1）涉及两个瓶颈。值得注意的是，自旋非限制计算预测了S-N键形成（通过模式1）的预反应复合物，该复合物已使用米勒的统一过渡态理论和双过渡态模型进行处理。本工作对反应中涉及的所有物种的命运进行了严格评估，所做的预测可能成为涉及自由基 - 自由基反应的进一步实验和理论研究的主题。
