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

Abstract

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 νOH stretching, the δOH bending and the τOH torsion around BO bond. Hot bands at 25°C, whose origin is the τOH level at 530 cm⁻¹, are revealed by cooling the sample at −173°C. A sample evacuated at 760°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)₂BOH because the 2τOH overtone is close to δOH, near 1030 cm⁻¹.