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 H-2(SiOH)(2), and some cage-like clusters of the hydridosilasesquisiloxane type with 4 to 8 Si atoms and one/two SiOH groups, respectively. The delta(H-1) is in good agreement with experiment with F3SiOH and cage-like models, whereas only these latter account satisfactorily for the delta(Si-29). H3SiOH and H-2(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. (C) 1999 Elsevier Science B.V. All rights reserved.