DFT study of zeolite LTA structural fragments: Double four-member rings ofoxygen-bridged silicon and aluminum atoms

The electronic structure and relative stability of [H8Si8-xAlxO12](x-) stru ctural fragments with Si and Al atoms located at tetrahedral sites and conn ected by bridging oxygen atoms so as to form a tetragonal prism, together w ith oxygen-coordinated charge-compensating cations, are studied by the B3LY P/G-31G(d) method. Clusters with Al/Si ratios in the 0-1 range and differen t orderings of the extraframework cations and Si and Al at tetrahedral site s are proper models for the double four-member rings, which an secondary bu ilding units of the LTA structure. The stability of the structure fragments is higher when the framework's negative charges are at maximum distance, i n agreement with Dempsey's rule. The HOMO orbitals of H8Si8O12, the anionic clusters [H8Si8-xAlxO12](x-), and the proton-balanced ones are mainly O 2p nonbonding AO. In fragments with Al/Si > 0 and Na+ at extraframework catio n positions, the oxygen nonbonding nature of the HOMO orbital decreases and certain electron density is shifted to the Al centers. The LUMO orbitals o f NaxH8Si8-xAlxO12 clusters consist of the charge-balancing cations' AO.