Ab initio studies of silica-based clusters. Part I. Energies and conformations of simple clusters

To understand the complex mechanisms of reaction, solvation, and diffusion that determine the chemistry of silica in solution, it is necessary to study first the silicate clusters that participate in these processes. We have investigated, by ab initio density functional methods, all silica-based clusters of the form SI(x)O(y)(OH)(z), with a maximum of five silicon atoms and two intramolecular condensations, plus the six-silicon ring and the eight-silicon containing cube. Tn this article (part I), we report our results on the structure, charge distribution, and energy of the simpler clusters: the monomer Si(OH)(4), the dimer Si(2)O(OH)(6), the linear trimer Si(3)O(2)(OH)(8), the ring trimer Si(3)O(3)(OH)(6), the linear tetramer Si(4)O(3)(OH)(10), the ring tetramer Si(4)O(4)(OH)(8), the linear pentamer Si(5)O(4)(OH)(12), and the cubic cage Si(8)O(12)(OH)(8). We also present density functional results for aluminosilicate clusters: Al(OH)(4)(-), Al(2)O(OH)(6)(2-), and SiAlO(OH)(6)(-). The results for the more complex clusters are presented in the subsequent article (part II). Our studies reveal a wide diversity of structures and consequently of charge distributions and energies for these clusters. that directly influence their chemical behavior, in particular the interaction with other clusters and with the solvent.