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 acuo at 750°C. A set of bands is assigned to overtone and combination transitions for normal modes involving mainly the SiOH group: the νOH stretching, the νSi–OH stretching, the δOH bending and the τOH 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 δOH bending is coupled with a symmetric stretching of SiOSi bridges in an (SiO)3SiOH unit. Pairs of bands involving some SiOH bending character (at 4510/4580 cm−1 for combinations with νOH and at 8100/8165 cm−1 for combinations with 2νOH) were extrapolated to fundamental transitions at 760/835 cm−1. This coupling was not observed for the deuterated sample because the δOD 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 τOH and its overtones, is also analysed.