Cage-like clusters as models for the hydroxyls of silica: ab initio calculation of 1H and 29Si NMR chemical shifts

NMR features of isolated and geminal silica hydroxyls were calculated using the B3-LYP hybrid density functional method, and adopting as models both the minimal clusters H3SiOH, F3SiOH and H2(SiOH)2, and some cage-like clusters of the hydridosilasesquisiloxane type with 4 to 8 Si atoms and one/two SiOH groups, respectively. The δ(1H) is in good agreement with experiment with F3SiOH and cage-like models, whereas only these latter account satisfactorily for the δ(29Si). H3SiOH and H2(SiOH)2 yield unsatisfactory results for both chemical shifts. The interaction with water molecules has also been considered, to investigate the effect of H-bonding on NMR features. As cage-like structures also account well for vibrational features [B. Civalleri, E. Garrone, P. Ugliengo, Chem. Phys. Lett. 294 (1998) 103] these molecules constitute the best available simple models for the surface hydroxyls of silica.