Spectroscopic and thermodynamic study of the H-bonding of olefins onto the isolated hydroxyl of amorphous silica

FTIR data are reported concerning the adsorption of ethene, propene, but-1-ene, trans-but-2-ene, cyclohexene and methylcyclohexene on Aerosil outgassed at 800 degrees C. H-bonding is the only process occurring with the four lighter molecules, and a strictly Langmuir-type behaviour is found. With the two heavier species, plain physisorption also takes place, causing the solvation of H-bonded adducts, which are best defined at low coverage. From the intensity of the nu(OH) mode of the free hydroxyls as a function of pressure, the equilibrium constant for the H-bond formation is calculated in different ways, according to the role of physisorption. Several correlations are drawn for Delta nu(OH) and Delta G degrees. The contribution of the substituents to the basicity of the double bond as measured by the shift in frequency is discussed, as well as the role of H-bonding and van der Waals forces in dictating the energetics of the interaction.