ABINITIO MOLECULAR-ORBITAL CALCULATED STATIONARY-POINTS ON THE POTENTIAL-ENERGY SURFACE OF H4ALO4- - SIMILARITIES WITH H4SIO4

Ab initio calculations of the stationary points on the potential energy surface for the H4AlO4- species were carried out at the Hartree-Fock level with the STO/6-31G* basis set. Full geometry optimisation with no symmetry constraint followed by harmonic frequency analysis predicted the existence of three minimum-energy structures. Self-consistent-field (SCF) calculations using diffuse functions were also performed in order to assess the importance of such functions for an adequate description of the anionic species H4AlO4-. The rich conformational flexibility observed is thought to be due to anomeric effects involving Al-O sigma* orbitals and vicinal oxygen tone pair p(pi) orbitals. It is shown that the conformational space occupied by H4AlO4- is similar to that occupied by H4SiO4. This provides a good explanation of the ease of substitution of aluminium for silicon atoms in aluminosilicate frameworks and, in particular, it provides a ready explanation for the ease of insertion of aluminium atoms into zeolite silicate frameworks during synthesis.