Electron paramagnetic resonance and infra-red data on H-associated centers in silica were analyzed by means of first-principles Hartree-Fock and density-functional theory calculations. The geometrical and electronic structures of the paramagnetic centers, (Si-O)3Si•, (Si-O)2Si•(OH) and (Si-O)2Si•(H), and of the corresponding diamagnetic centers which were formally derived from the addition of one H atom, (Si-O)3Si(H), (Si-O)2Si(OH)(H) and (Si-O)2Si(H)(H), were determined by using cluster models. Substantial agreement was found between the computed hyperfine splittings, and those measured for bulk paramagnetic centers in high-purity amorphous silica. The different experimental hyperfine splittings which were obtained for (Si-O)3Si• centers at the surface of mechanically or thermally activated silica were explained in terms of small structural differences with respect to the bulk counterparts. Almost quantitative agreement with experiment was found for the vibrational properties of the Si-H groups of the silylhydride centers. The results permitted the definite attribution of spectral features to specific point defects in silica.

EPR and IR Spectral Properties of Hydrogen-Associated Bulk and Surface Defects in SiO2: ab initio Calculations. G.Pacchioni, M.Vitiello: Physical Review B, 1998, 58[12], 7745-52