气相金属簇阳离子Pd_2^+和Pd_3^+活化甲烷分子中C-H键的理论研究

Theoretical Investigation for Activation of C-H Bond in Methane by Pd_2^+ and Pd_3^+ in the Gas Phase

通过使用密度泛函理论B3LYP的方法在二重态及四重态势能面上研究了金属簇阳离子Pd+2和Pd+3活化甲烷的机理.使用局部优化的方法确定了高自旋与低自旋势能面之间的交叉点,同时,通过自旋轨道耦合值的计算研究了MECP点可能的自旋翻转过程.根据Landau-Zener公式计算出的电子跃迁几率,我们发现在MECP点发生了有效的系间窜越.计算表明Pd+2体系以自旋保守的方式反应在基态势能面上.得到的最终产物是卡宾和氢气,反应吸收11.84kcal·mol-1的热量.Pd+3体系是两态反应,最终的反应产物是氢桥键分子和氢气,反应放热9.05kcal·mol-1.研究表明Pd+3在室温下能自发地活化甲烷,而Pd+2活化甲烷在热力学和动力学上都是不利的过程,计算结果与实验的观察结果一致.

The mechanisms of methane activation catalyzed by small cationic palladium clusters Pd＋2 and Pd＋3 have been investigated by using hybrid density functional B3 LYP method.Both the quartet and doublet potential energy profiles have been considered.Crossing points between the high-spin and low-spin potential energy surfaces are located using local optimization method.At the same time,possible spin-flip processes at MECP are investigated through spin-orbit coupling（SOC）calculations.According to the hopping probability calculated with the Landau-Zener formula,effective intersystem crossing might happen at each MECP.Calculations indicate that the Pd＋2 system reactions proceed on the ground-state potential energy surface with a spinconserving manner.The final products are metal-carbene and hydrogen and the reaction require endothermic by 11.84k cal·mol-1.While the Pd＋3 system is a two-state reactivity（TSR）,mechanism and the final products are hydrogen-bridge bond molecules.The reaction need exothermic by 9.05k cal·mol-1.Finally,study shows that Pd＋3 activate of methane spontaneously at room temperature,but Pd＋2 with methane are both thermodynamically and kinetically unfavorable processes.The calculated results are consistent with the experimental observations.