Vibrational properties of surface hydroxyls: Nonempirical model calculations including anharmonicities

Abstract

Complete sets of harmonic, semidiagonal cubic as well as diagonal cubic and quartic force constants are reported for the internal coordinates of terminal, ≣SiOH, and bridging, ≣SiOH·Al≣, surface hydroxyls on silica and zeolites. They are obtained by numerical differentiation of analytically calculated gradients of the energy (SCF approximation, 6-31 G* basis set). A GF vibrational analysis is performed and after making a nonlinear transformation of the force constants into normal coordinates the anharmonicity constants are evaluated by perturbation theory. Comparison is made with the D₂OH⁺ ion and the DOH molecule. The calculated anharmonicities of the OH bonds in the systems studied are remarkably constant and vary between -76 and -84 cm^-1, only in agreement with the values observed for DOH (-83 cm^-1) and surface silanols, ≣SiOH (-90 ± 15 cm^-1).