SPECTROSCOPIC AND ABINITIO STUDY OF THE INTERACTION OF MOLECULAR-HYDROGEN WITH THE ISOLATED SILICA HYDROXYLS AND RELATED SYSTEMS

Experimental vibrational data concerning the interaction of dihydrogen 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, one envisaging dihydrogen as a proton donor to the oxygen atom in SiOH (structure F), the other as a proton acceptor in a T-shaped structure (structure T). Calculated properties are the binding energy, frequencies of vibrational motions in the harmonic approximation, and H-H infrared intensity in the double-harmonic approximation. Both structures are stable. Structure T is more weakly bound and has less IR active H-H stretch than structure F, whose calculated features are in better agreement with the experiment. Experimental results concerning the bridging hydroxyl of H-mordenite are also reported. Structures F and T are compared with the known gas-phase complexes of molecular hydrogen.