Mo活化NH_3的自旋禁阻反应机理

Spin-Forbidden Reaction Mechanism of NH_3 Activated by Mo Atom in Gas Phase

采用密度泛函理论的UB3LYP方法,计算研究了气相中Mo活化NH3的反应机理。为了理解由Mo活化NH3过程中自旋翻转行为,对自旋态分别为七、五、三和单等4个反应势能面进行了计算研究,其结果表明,Mo活化NH3的过程是通过各自旋态势能面交叉产生的典型的自旋禁阻反应,最低能量交叉点(MECPs)附近的系间窜越导致2步H转移和脱H2反应能垒降低。此外运用自然键(NBO)轨道理论分析了反应中较为重要的几个物种的成键特性。通过计算在最低能量交叉点(MECPs)附近不同自旋态之间的自旋-轨道耦合常数,再运用Landau-Zener跃迁几率公式估算了MECPs处系间窜越几率。所确定的最低能量反应路径为:7Mo+NH3→7IM1→7/5MECP1→5TS12→5IM2→5/3MECP2→3TS23→3IM3→3TS34→3IM4→3HMoN+H2。

Gas-phase ammonia activation mechanism by Molybdenum（Mo） atoms was studied at the density functional level of theory using the hybrid exchange correlation functional B3LYP.Four reaction potential energy surfaces（PESs）corresponding to the septet,quintet,triplet,and singlet multiplicities were investigated calculationally in order to understate the presence of some spin inversion during ammonia activation.The results showed that ammonia activation mediated by Mo atoms is a typical spin-forbidden process resulting from the crossing among the multistate PESs.The minimum energy crossing points（MECPs） lead to decrease in the barrier heights of TS12 and TS23 that correspond to the first,second hydrogen transfer and the reductive elimination step of H2,respectively.We also made use of the natural bond orbital（NBO） theory to characterize the bonding characters between different groups for some important species involved.The spin-orbit coupling is calculated between electronic states of different multiplicities at MECPs to estimate the intersystem crossing（ISC） probabilities,and the ISC probability of hopping from one surface to the other in the vicinity of the crossing region is calculated by the Landau-Zener type model.The minimum energy pathway can be described as 7Mo＋NH3→7IM1→7/5MECP1→5TS12→5IM2→5/3MECP2→3TS23→3IM3→3TS34→3IM4→3HMoN＋H2.