Modelling the Structure and Properties of Aluminophosphonates
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Modelling the Structure and Properties of Aluminophosphonates Paramjit Grewal 1, Paul A Wright2, Mark Edgar2, Julian D Gale3 and Paul A Cox1 1 2 3
Centre for Molecular Design, University of Portsmouth, King Henry Building, King Henry I Street, Portsmouth, PO1 2DY, UK. School of Chemistry, University of St. Andrews, The Purdie Building, North Haugh, St. Andrews, Fife, KY16 9ST, UK. Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, SW7 2AY, UK.
ABSTRACT Several aluminophosphonate materials have been investigated using both semi-empirical quantum mechanical and Density Functional Theory (DFT) methodologies. The optimised structures obtained are in excellent agreement with experimental results. Important information on the electronic distribution in these structures is obtained, allowing charge distributions to be determined and H2O-framework interactions to be probed. The barriers to rotation for the organic groups in three structures have been investigated. Results for –(CH3) groups in AlMePO-α and AlMePO-β, yield barrier heights that are consistent with rapid rotation at ambient temperature, whereas the barrier height obtained for –(C6H5) in AlBzPO-I suggests that the framework will significantly hinder rotation. The use of modelling to help elucidate the structure of a novel compound, AlMePO-2, and to probe the structure and stability of a hypothetical aluminium ethyl phosphonate, AlEtPO, are also illustrated. INTRODUCTION Metal phosphonate materials are of considerable interest because they contain both inorganic and organic groups covalently bound in the framework. They therefore potentially possess complementary adsorptive and catalytic properties to zeolitic materials, with potential applications in diverse areas such as sorption, ion exchange and catalysis. Aluminium phosphonates, such as aluminium methylphosphonates, benzylphosphonates, phenylphosphonates , carboxylethylphosphonates and diphosphonates are shown to have a range of structure types, which are typically based on layered arrangements of the organic and inorganic groups. The exceptions to these structural arrangements are two polymorphs of Al2 (CH3PO3) 3 (AlMePO-α and AlMePO -β). Their frameworks consist of hexagonal arrays of one-dimensional channels lined with methyl groups. The mixed inorganic-organic nature of the aluminophosphonates means that they present a notable challenge to theoretical methods because the unusual chemical environment adopted by some atoms makes it more difficult to derive reliable parameters for theoretical methods based on interatomic potentials. In this study, the aluminophosphonate structures AlMePO-1 [1], AlMePO-2 [2], AlMePO-α[3], AlMePO-β [4], AlMePO-γ [5], AlMePO-zeta [6] and AlBzPO-I [7] have been modelled using both semi-empirical quantum mechanical and Density Functional Theory (DFT) methodologies. These methods have an advantage over molecular mechanics and related methods in that no parameters have to be derived for the particular structures under investi
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