Effect of heat treatment on boron impurity in Vycor.: Part I.: Near infrared spectra and ab initio calculations of the vibrations of model molecules for surface boranols

Near infrared spectra of surface boranol groups on a preheated Vycor silica glass were observed and interpreted with the help of ab initio calculations of the vibrational properties of BOH groups anchored to silica or to B2O3 clusters. The spectra of deuterated samples were also obtained. A set of narrow lines corresponds to overtone and combination transitions for normal modes involving mainly the hydroxy group, that is the nu OH stretching, the delta OH bending and the tau OH torsion around BO bond. Hot bands at 25 degrees C, whose origin is the tau OH level at 530 cm(-1), are revealed by cooling the sample at -173 degrees C. A sample evacuated at 760 degrees C displays mainly isolated boranol groups and some very weakly perturbed species, likely vicinal groups, bonded to silica. A thorough analysis of the anharmonic properties of isolated boranol is given. The spectrum assignment is extrapolated to transitions hidden by the very strong absorptions of the bulk material. Fermi resonances appear in diads for species (SiO)(2)BOH because the 2 tau OH overtone is close to delta OH, near 1030 cm(-1).