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Effects of complexation of dimethyl phosphate anion (DMP-) with countercations Li+, Na+, K+, Mg2+, and Ca2+ on geometry, charge distribution, IR spectra, and conformational changes of DMP- are investigated using DFT calculations. IR spectra alterations caused by conformational change are shown to depend on the ionic strength of a cation and environment.

Adsorption of the phosphate groups on a silica surface is investigated by ab initio calculations of hydrogen-bond complexes of dihydrogen and dimethyl phosphate anions with orthosilicic acid. It is found that the phosphate groups can form strong hydrogen-bonded complexes with the silanols of the silica surface with stabilization energies of ca. −14 kcal/mol per hydrogen bond. The effect of the coordination of the phosphate species by water, orthosilicic acid, and a sodium cation on the geometry and stability of the -sc/-sc conformation is investigated through consideration of 2-fold axial symmetry complexes. Calculations suggest the reduction of the COPO torsion angle of the -sc/-sc conformation of the phosphodiester backbones upon adsorption of molecules containing these moieties on silica compared to the value of this angle found for phosphates in vacuo and in hydrated forms. Both the polarizing and electron density abstracting abilities of silanols cause electron density redistribution, giving rise to ...

A quartz family of silica polymorphs is studied. The structure and phase transformations of quartz, moganite, and orthorhombic silica ($o$-silica) are examined using molecular dynamics simulations. Moganite and its high-pressure forms appear to be the most stable phases in the temperature range from 100 to 1100 K and the pressure range from 0 to 30 GPa. Upon applied pressure $o$-silica transforms first into a monoclinic phase (at about 15 GPa) and then into a high-density orthorhombic phase (at about 28 GPa). Moganite also exhibits two crystal-crystal phase transformations, at about 5 and 21 GPa. Quartz undergoes amorphization at approximately 21 GPa in accordance with previous studies. Moganite and $o$-silica are likely to be present among naturally occurring microcrystalline forms of silica; however, similarities in their x-ray-diffraction patterns would make it very difficult to distinguish between them. The implications of computer simulation results for crystal growth and metamorphism are discussed.

The MP2(fc)/6-31G∗ structures, energies and frequencies of the trisilaallyl cation (1) and a number of its Si3H5+ isomers reveal considerable differences from the C3H5+ potential energy surface. Unlike the allyl cation, 1 is not a minimum. A “quasi-cyclic” form (7), with a much shorter Si(1)Si(3) separation and aromatic character, is the global Si3H5+ minimum at MP2 and at B3LYP/6-311+G∗∗. However, Raghavachari's (J. Chem. Phys., 95 (1991) 7373) double-bridged isomer (5) is 1.4 kcal mol−1 lower in energy than 7 at the CCSD(T)/6-311+G ∗ //MP2/6-316 ∗ level. Conjugation in the planar C2v 1 and hyperconjugation in the rotated 9 form are decreased relative to the carbon analogs. The remarkable stability of 5 and 7 can be attributed to their effective charge delocalization.

A transformation of the quartz-type ${\mathrm{GaPO}}_{4}$ to a new high-pressure phase has been observed in molecular dynamics simulation at 7 GPa (experimental value is 9 GPa). The phase has a monoclinic P${2}_{1}$ structure and can be viewed as a wafer structure composed of more densely packed layers with six-coordinated Ga separated by less dense layers containing five-coordinated Ga. Phosphorous retains its fourfold coordination. Upon decompression this new monoclinic phase does not convert back to the quartz-type structure, but undergoes structural deformation without a change in the space-group symmetry. Prolonged annealing of this phase at 1000 K recovered the quartz-type structure. \textcopyright{} 1996 The American Physical Society.