Silanol as a model for the free hydroxyl of amorphous silica: Ab initio calculations of the interaction with ammonia

Literature experimental data, both vibrational and energetic, concerning the interaction of ammonia 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. Two modes of interaction are considered. envisaging ammonia as a proton donor (I) and as a proton acceptor (II). Calculated properties are: the binding energy; frequencies and intensities of vibrational motions in the harmonic approximation; enthalpy, entropy and free enthalpy standard changes of reaction and related adsorption isotherm. Structure I is very weakly bound and unstable. The calculated enthalpy of adsorption and the vibrational features of structure II are in remarkable agreement with the experiment: calculated entropy, free enthalpy changes and adsorption isotherm are in error. because of the inadequacy of the rigid cluster model used in the computation of these latter. Electrostatic, dispersive and charge-transfer contributions to the binding energy are discusse