A DFT quantum-chemical study on the structures and active sites of polymethylaluminoxaneстатья

Статья опубликована в высокорейтинговом журнале

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Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.

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[1] A dft quantum-chemical study on the structures and active sites of polymethylaluminoxane / I. I. Zakharov, V. A. Zakharov, A. G. Potapov, G. M. Zhidomirov // Macromolecular Theory and Simulations. — 1999. — Vol. 8, no. 3. — P. 272–278. The electronic structure and geometry of polymethylaluminoxane (MAO) [-Al(CH3)O-]n with different size (n = 4-12) have been studied using quantum-chemical DFT (density functional theory) calculations. It has been found: 1) Starting from n = 6, the three-dimensional oxo-bridged (cage) structure of MAO is more stable than the cyclic structure. 2) Both for cage structure and for cyclic structure the Lewis acidity of Al atoms characterized by their net positive charge amplifies with increasing size of MAO (n). 3) Trimethylaluminium (AlMe3) reacts with the cage structure of MAO with cleavage of an Al-O dative bond and formation of acidic tri-coordinated Alv and basic di-coordinated Ov atoms in the MAO molecule. Two molecules AlMe3 are associated with acidic Alv and basic Ov centers. As the MAO increases in size, the acidity of Alv centers amplifies and the distance Alv-(AlMe3) shortens; on the contrary, interaction of AlMe3 with Ov centers weakens and the distance Ov-(AlMe3) increases with increasing n value. The total heat of Al2Me6 interaction with MAO (sum interaction of Alv-(AlMe3) and Ov-(AlMe3)) noticeably decreases as the size of MAO increases (from 50.9 kcal/mol for n = 4 to 20.2 kcal/mol for n = 12). It is proposed that acidic Alv and basic Ov centers formed in the cage structure of MAO interact with zirconocene yielding ’cation-like’ zirconium active centers. © Wiley-VCH Verlag GmbH, 1999.

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