CH_3与HO_2自由基反应途径及位垒的计算研究

Computational Study of Potential Barrier and Reaction Paths of CH_3 with HO_2

采用量子化学从头算中的QCISD方法,在cc-pvdz和cc-pvtz基组水平上对CH3+HO2反应机理进行了计算研究,结果表明,CH3与HO2双分子自由基反应是一个复杂反应,反应可以在不同的电子态下经数个产物通道进行.在HO2自由基上的H直接转移到甲基自由基生成CH4+O2的反应途径上,存在一个低于反应物能量8.49 kJ/mol的分子复合物.直接氢转移反应的计算位垒为3.07 kJ/mol,与实验值零位垒相近.由CH3OOH生成CH2O和H2O反应的过渡态呈四元环构型,且具有Cs对称性,基于IRC计算,证明了该过渡态引导CH3OOH生成CH2O和H2O的反应机理是一个分子内氢迁移和H2O消除的协同过程.对由反应物生成CH3O和OH自由基的反应,除存在协同机理外,还提出了另一可能的通道,即反应物沿单线态位能面经过一个分步反应完成,也即反应物先经无位垒过程产生CH3OOH及其异构体CH3(O)OH,其中CH3(O)OH发生解离反应生成CH3OH和单线态原子氧,然后原子氧转化为三线态再与CH3OH进行反应生成CH3O和OH.

Quantum chemistry calculation methods were employed to analyze reaction mechanism of reaction CH_3+HO_2. Results indicated that the reaction is a complex one because reaction occur on the several electron states and have several products channels. Possible stationary points, on the CH_3+HO_2 reaction potential surface had been studied using QCISD and QCISD (T) method at cc-pvdz and cc-pvtz level. In the reaction process of formation CH_4+O_2, there is a molecular complex called vdW, which lies 8.49kJ/mol below the reactants with a 3.07kJ/mol barrier leading to the formation of CH_4 and O_2 (~3Σ_g); In the reaction of formation of H_2O+CH_2O, the transition state geometry is a four-member ring and has Cs symmetry, and IRC calculation confirmed the transition state lead a H transfer and H_2O elimination mechanism for formation H_2O+CH_2O by CH_3OOH. Formation of CH_3OOH and CH_3(O)OH are barrier-less processes, and two well-defined isomerization transition states TS3 and TS3~* that link CH_3OOH and CH_3(O)OH through different mechanism were reported first, and the oxygen decomposition of isomeric CH_3(O)OH would help us explain how CH_3O and OH formed through a stepwise mechanism.