Silanol as a model for the free hydroxyl of amorphous silica: non-empirical calculations of the vibrational features of H3SiOH

Amorphous silica after thermal treatment at high temperature exhibits isolated OH groups capable of interaction with a variety of molecules. Due to the covalency of the matrix, a cluster approach to the SiOH behaviour is particularly suitable. The simplest cluster model is H3SiOH (silanol). H3SiOH does not exist as such as it condenses to siloxane. The suitability of silanol as a model compound is thus assessed in the present work by comparing calculated values of the torsional barrier, the full set of vibrations in the harmonic approximation, and the anharmonic OH stretch (including overtones) with the corresponding experimental values for silica hydroxyls. All the calculations have been carried out at ab-initio level employing basis sets larger than in the past, and including electron correlation through perturbative technique. The agreement with experiment is good already at the Hartree-Fock level, and it further improves by inclusion of the electronic correlation and anharmonicity effects.