FeCH2＋H2→Fe＋CH4反应机理的密度泛函理论研究

Theoretical Study on the reaction of FeCH_2+H_2→Fe+CH_4

采用密度泛函理论(DFT)中的UB3LYP方法在6-311+G(2d,2p)水平上研究了五重态和三重态的FeCH2与H2反应的机理,在UB3LYP结构优化的基础上,用耦合簇理论方法UCCSD(T)在相同水平下对各驻点进行了单点能校正.结果表明,该反应在三重态和五重态上的势能面非常相似,存在两个过渡态(TS1、TS2)和两个中间体(M1、M2).反应的第一步是产生分子复合物(H2)FeCH2,其能量分别比反应物低16.6(五重态)和20.3kJ/mol(三重态);第二步是H2的加成形成HFeCH3中间体,第三步是CH4的消除反应.其中CH4的消除为整个反应的速控步骤.H-H键的活化在五重态上是不可逆的,反应将最终形成五重态产物,整个反应可放出能量300.6kJ/mol.

In the present paper,UB3LYP methods compiled with the 6-311＋G（2d,2p） basis set were employed to investigate the mechanism of the reaction of FeCH2（triplet and quintet） and H2.Based on B3LYP/6-311＋G（2d,2p） geometries,single point UCCSD（T） energy calculations have been performed to get more accurate energies.The PES of the reaction,calculated from triplet and quintet states of FeCH2,are very similar.In the first step,the reactants give an molecule complex,（H2）FeCH2,with a stabilization energy of 16.6（quintet） and 20.3（triplet） kJ/mol.There exist a cross point in this process on the PES.Then the H-H bond activation takes place and followed by methane elimination from HFeCH3.The methane elimination was predicted to be rate-controlling step for both triplet and quintet reaction.The H-H bond activation for the quintet state is irreversible and the activation energy is smaller than that in triplet state reaction.So the FeCH2＋H2 reaction is mainly completed through quintet state.