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

Literature vibrational data concerning the interaction of formaldehyde with the isolated hydroxyl of amorphous silica are compared with the results of ab initio calculations by both the Hartree-Fock and correlated through perturbative technique (MP2 and MP3). Silanol H3SiOH is chosen to mimic the silica free hydroxyl: H2CO acts as H-acceptor. Calculated properties are the binding energy, vibrational spectrum in the harmonic approximation and free enthalpy of interaction. Excellent agreement with experiment is found. To characterize the intermolecular motions, a significant section of the intermolecular-potential hypersurface has been calculated; these results are so extremely flat as to render difficult the definition of proper normal modes. The comparison with the appropriate gas-phase complexes of formaldehyde shows that the acidity of H3SiOH is intermediate between HCl and HF