SILANOL AS A MODEL FOR THE FREE HYDROXYL OF AMORPHOUS SILICA - ABINITIO CALCULATIONS OF THE INTERACTION WITH WATER

Literature experimental data, both vibrational and energetic, concerning the interaction of water with the isolated hydroxyl of amorphous silica are compared with the results of ab initio calculations both Hartree-Fock and correlated through perturbative technique (MP2). Silanol, H3SiOH, is chosen to mimic the silica free hydroxyl. Three modes of interaction are considered, envisaging water as a proton donor (I), as a proton acceptor (II), and in a bifurcated configuration. Calculated properties are the binding energy; frequencies and intensities of vibrational motion in the harmonic approximation; anharmonic SiO-H motion, including overtones; enthalpy, entropy, and free energy standard changes of reaction and related adsorption isotherm. The bifurcated configuration is very weakly bound and unstable. Structure I has a nonplanar stable configuration; structure II is, however, the most stable. Electrostatic, charge transfer, and dispersive contributions to the binding energy are discussed. Thermodynamic considerations indicate that the former may occur as a few percent of the total coverage. Enthalpy of adsorption and vibrational features are in gross accord with the experiment; calculated adsorption isotherms are definitely in error, due to the inadequacy of the rigid-cluster model used in the computation of these latter. The behavior of silanol toward the water dimer is also discussed. © 1990 American Chemical Society.