Kassab Emile, Castellà-Ventura Martine
Laboratoire de Chimie Théorique, CNRS-UMR 7616, Université Pierre et Marie Curie, Tour 22-23, Case 137, 4 place Jussieu, 75252 Paris Cedex 05, France.
J Phys Chem B. 2005 Jul 21;109(28):13716-28. doi: 10.1021/jp040524b.
The interactions of pyridine and 4,4'-bipyridine with the Lewis acid sites of alumina surfaces are investigated using ab initio and density functional calculations. Four cluster models of different sizes and shapes are chosen to represent the Lewis acid sites: three hydrogenated clusters Al(OH)(3), Al(4)O(9)H(6), and Al(10)O(21)H(12) and one non-hydrogenated cluster Al(4)O(6). The Hartree-Fock (HF) and B3LYP approaches with two basis sets 6-31G and 6-31+G are used to calculate the geometries, the electronic structures, the vibrational frequencies, and the adsorption energies of the complexes formed upon interaction of pyridine or 4,4'-bipyridine ligands on the cluster surfaces. Electronic structures are determined by the electrostatic potential (ESP) analysis of charges. Adsorption energies are calculated with corrections made for zero-point energies (ZPE) and basis set superposition error (BSSE). The ESP analysis of atomic charges reveals that the charge-transfer effects are more important in Lewis complexes formed with Al(4)O(6) cluster than in those formed with hydrogenated clusters Al(OH)(3), Al(4)O(9)H(6), and Al(10)O(21)H(12). The significantly larger charge transferred from pyridine or 4,4'-bipyridine ligand to Al(4)O(6) cluster should increase the adsorption energy of these complexes. Consequently, at all levels of calculation, the adsorption energies of pyridine and 4,4'-bipyridine complexed to Al(4)O(6) cluster ( approximately 46 kcal/mol), which compare very well to experiment, are strongly larger than those obtained for both pyridine and 4,4'-bipyridine ligands complexed to Al(OH)(3) (32 kcal/mol), Al(4)O(9)H(6) (24 kcal/mol) and Al(10)O(21)H(12) (25 kcal/mol) clusters. The corrected adsorption energy is found to be insensitive to basis set and electron correlation effects. It essentially depends on the ionic character of the cluster model rather than on its size. For 4,4'-bipyridine complexes, similar results to those obtained for pyridine are found, and the geometry and the amount of charge of the unbound pyridyl ring are unchanged upon complexation. The calculated vibrational frequencies and frequency shifts are little sensitive to the size and shape of the cluster model. The two ring stretching modes 8a and 19b of pyridine and 4,4'-bipyridine observed in the 1400-1600 cm(-1) region are the most affected modes upon adsorption, in good agreement with the available infrared and Raman data.
采用从头算和密度泛函计算方法研究了吡啶和4,4'-联吡啶与氧化铝表面路易斯酸位点的相互作用。选择了四种不同大小和形状的团簇模型来代表路易斯酸位点：三种氢化团簇Al(OH)(3)、Al(4)O(9)H(6)和Al(10)O(21)H(12)以及一种非氢化团簇Al(4)O(6)。使用具有6-31G和6-31+G两种基组的哈特里-福克（HF）和B3LYP方法来计算团簇表面上吡啶或4,4'-联吡啶配体相互作用形成的配合物的几何结构、电子结构、振动频率和吸附能。通过电荷的静电势（ESP）分析确定电子结构。吸附能通过对零点能（ZPE）和基组叠加误差（BSSE）进行校正来计算。原子电荷的ESP分析表明，与Al(4)O(6)团簇形成的路易斯配合物中的电荷转移效应比与氢化团簇Al(OH)(3)、Al(4)O(9)H(6)和Al(10)O(21)H(12)形成的配合物中的电荷转移效应更重要。从吡啶或4,4'-联吡啶配体转移到Al(4)O(6)团簇的电荷明显更多，这应该会增加这些配合物的吸附能。因此，在所有计算水平上，与Al(4)O(6)团簇络合的吡啶和4,4'-联吡啶的吸附能（约46 kcal/mol）与实验结果非常吻合，比与Al(OH)(3)（32 kcal/mol）、Al(4)O(9)H(6)（24 kcal/mol）和Al(10)O(21)H(12)（25 kcal/mol）团簇络合的吡啶和4,4'-联吡啶配体的吸附能大得多。发现校正后的吸附能对基组和电子相关效应不敏感。它基本上取决于团簇模型的离子特性而不是其大小。对于4,4'-联吡啶配合物，发现了与吡啶类似的结果，并且未结合的吡啶环的几何结构和电荷量在络合后不变。计算得到的振动频率和频率位移对团簇模型的大小和形状不太敏感。在1400 - 1600 cm(-1)区域观察到的吡啶和4,4'-联吡啶的两个环伸缩模式8a和19b是吸附时受影响最大的模式，与现有的红外和拉曼数据非常吻合。
