咪唑在银表面吸附的理论分析

Theoretical Analysis of Adsorption of Imidazole on Ag (1, 1, 1) Surface

本文用EHMO紧束缚方法计算了咪唑在银(1,1,1)面上吸附的四种构型和吸附态的电子结构,得到咪唑在银表面上的优化构型为直立桥位吸附。按该方式,咪唑环上吡啶型氮原子的p轨道和邻近Ag原子s轨道之间形成多中心σ键。咪唑环上各原子的电荷布居在吸附后有较大的变化,表现出电子由银表面向咪唑转移,并进而使得环上N—H键解离能从吸附前的519.4kJ·mol^(-1)降低到吸附后的70.34kJ·mol^(-1)。这与实验中观察到的眯唑N—H键在Ag表面极易断裂的事实相符。

Four adsorption geometries and their electronic structures of imidazole on Ag (1, 1, 1) surface have been calculated by EHMO tight binding method. The Ag surface was represented by a monolayer slab. Results showed that the most favored configuration was a standing-up fashion at bridge sites by which the imidazole ring bound to metallic silver via its pyridine-tpye nitrogen lone pair electrons, therefore forming a multi-center sigma bonding between the p orbital of nitrogen and the s orbitals of the adjacent Ag atoms. The electron density changes before and after adsorption indicated that some of the electrons flew from silver into a pi antibonding molecular orbital of chemisorbed imidazole. Further, the calculations indicated that imidazole was susceptible to dissociate the hydrogen atom on pyrrole-type nitrogen when bound to clean Ag, implying the facile cleavage of N—H bond on Ag surface in agreement with the experimental facts.