ISSN:
0020-7608
Keywords:
Chemistry
;
Theoretical, Physical and Computational Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The structural, electronic, vibrational, and topologic properties of a series of acid sites of zeolites were studied at different levels of ab initio molecular orbital theory. The zeolite acid sites were modeled by using the following molecular clusters: silanol H3SiOH (B0) and the clusters H3SiO(H)AlH3 (B1), (OH)3SiO(H)Al(OH)3 (B1―OH), and H3SiO(H)Al(OH)2SiH3(B2). The calculation of geometries and properties of these clusters were performed at the Hartree-Fock level, and, additionally, second-order Møller-Plesset (MP2) and density functional BLYP calculations were carried out for silanol and B1 clusters. Geometries were fully optimized by following Cs symmetry restrictions. The standard STO/6-31+G(D, P) basis set, which includes polarization and diffuse functions, was used for all the calculations. The topologic properties of the zeolite acid clusters, based on the theory of atoms in molecules, were analyzed in terms of the total density and the Laplacian density properties, both evaluated at the position of the bond critical points. The calculations showed that the frequency of the OH vibrational modes of the zeolite acid sites, often used as an infrared index for characterizing the acidity of zeolites, is linearly related to the total density of the charge at the critical points of the OH bonds, with a correlation coefficient of r2=0.97. These results indicate that the total density of the electronic charge at the critical point of the OH bond can be used as a tool for interpreting the structural and electronic features of the zeolite hydroxyl groups. A relationship between the Mulliken population of the H atom of the OH bond and the OH frequency gives a correlation coefficient of 0.67. On the other hand, the values of the Laplacian density calculated at the critical points of the bonds of the acid sites indicate that the zeolite structure is dominated by a network of Si—O and Al—O ionic interactions, while the O(SINGLE BOND)H bonds are characterized as covalent bonds, with different extents of charge concentration. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 951-960, 1998
Additional Material:
4 Ill.
Type of Medium:
Electronic Resource
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