Effect of heat treatment on boron impurity in Vycor. Part I. Near infraredspectra 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 calculation s 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 n arrow 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 de grees C, whose origin is the tau OH level at 530 cm(-1), are revealed by co oling the sample at -173 degrees C. A sample evacuated at 760 degrees C dis plays mainly isolated boranol groups and some very weakly perturbed species , likely vicinal groups, bonded to silica. A thorough analysis of the anhar monic properties of isolated boranol is given. The spectrum assignment is e xtrapolated to transitions hidden by the very strong absorptions of the bul k material. Fermi resonances appear in diads for species (SiO)(2)BOH becaus e the 2 tau OH overtone is close to delta OH, near 1030 cm(-1).