CH3S与HCS双自由基反应的密度泛函理论研究

应用量子化学从头算和密度泛函理论(DFT)对CH₃S与HCS双自由基单重态反应进行了研究. 在MPW1PW91/ 6-311G(d,p)水平上优化了反应通道上各驻点(反应物、中间体、过渡态和产物)的几何构型, 用内禀反应坐标(IRC)计算和频率分析方法对过渡态进行了验证. 在QCISD(t)/6-311＋＋G(d,p)水平上计算各物种的单点能, 并对总能量进行了零点能校正. 研究结果表明, CH₃S与HCS反应为多通道反应, 有4条可能的反应通道, 反应物首先通过S…S弱相互作用形成具有竞争反应机理的五元环硫-硫偶合中间体a和链状硫-硫偶合中间体c, 再由此经过氢迁移、离解、异构化等不同机理得到主要产物P1 (2CH₂S), 次要产物P2 (CH₃SH＋CS), P3 (CH₄＋CS₂)和P4 CH₂(SH)CSH]. 根据势能面分析, 所有反应均为放热反应, 生成P1的反应热为－165.55 kJ?mol^－1. 通道R→a→TSa/b→b→P1为标题反应的主通道, 其速控步骤a→TSa/b→b在200～2000 K温度区间内的速率常数可以表示为k₁^CVT/SCT＝1.75×10¹⁰T^0.65exp(－907.6/T) s^－1. P3及P4的生成需要越过很高的活化能垒, 是动力学禁阻步骤, 但在反应体系中加入合适催化剂, 改变其反应机理, 有可能使生成CH₂(SH)CSH, CH₄及CS₂的反应易于进行.

Density Functional Theory Study on the Biradical Reaction between CH3S and HCS

A detailed theoretical survey of the potential energy surface (PES) for the reaction CH₃S with HCS in gas phase is carried out at the QCISD(t)/6-311＋＋G(d,p)//MPW1PW91/6-311G(d,p) level. The geometries, vibrational frequencies, and energies of all stationary points involved in the title reaction are calculated at the MPW1PW91/6-311G(d,p) level. More accurate energy information is provided by single-point calculations at QCISD(t)/6-311＋＋G(d,p) level. Relationships of reactants, intermediates, transition states and products are confirmed by the intrinsic reaction coordinate (IRC) calculations. 8 intermediates and 9 transition states are located and a variety of possible reaction pathways are probed. The association of CH₃S with HCS is found to be a barrierless process. Firstly it forms the energy-rich adducts a (five-member ring-type structure) or c (chain structure) through the weak S…S bond. Then, from adducts a and c, the main product P1 (2CH₂S) can be produced, as well as secondary important products P2 (CH₃SH＋CS), P3 (CH₄＋CS₂) and P4 CH₂(SH)CSH] via the different channels, i.e. hydrogen shift, dissociation and isomerizazion. The reaction pathway leading to the major product 2CH₂S is as follows: R→a→TSa/b→b→P1. The rate constant of step a→TSa/b→b is k₁^CVT/SCT＝1.75×10¹⁰T^0.65exp(－907.6/T) s^－1 by means of small-curvature tunneling correction in the temperature range of 200～2000 K. By the analysis of the potential energy surface, we can draw the conclusion that all the channels are exothermic reactions and the reaction heat of generating P1 is －165.55 kJ?mol^－1.