Near infrared and ab initio study of the vibrational modes of isolated silanol on silica

Near infrared spectroscopy was used to study the vibrational modes of isolated silanol groups in porous silica monoliths that had been heated in vacuo at 750 degrees C. A set of bands is assigned to overtone and combination transitions for normal modes involving mainly the SiOH group: the nu OH stretching, the nu Si-OH stretching, the delta OH bending and the tau OH torsion around the SiO bond. The spectra of a deuterated sample were also analysed. The wavenumbers for the fundamental transitions were deduced and some anharmonicity coefficients were calculated. The delta OH bending is coupled with a symmetric stretching of SiOSi bridges in an (SiO)(3)SiOH unit. Pairs of bands involving some SiOH bending character (at 4510/4580 cm(-1) for combinations with nu OH and at 8100/8165 cm(-1) for combinations with 2 nu OH) were extrapolated to fundamental transitions at 760/835 cm(-1). This coupling was not observed for the deuterated sample because the delta OD absorption at 605 cm(-1) is well separated from SiOSi stretching centred at 815 cm(-1). The experimental spectra are discussed with the help of ab initio calculations of the vibrational modes of a molecule modelling an isolated silanol group on silica. The contribution of hot transitions at room temperature, from low lying levels such as tau OH and its overtones, is also analysed.