Spectroscopic and thermodynamic study of the H-bonding of olefins onto theisolated 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 outgasse d at 800 degrees C. H-bonding is the only process occurring with the four l ighter molecules, and a strictly Langmuir-type behaviour is found. With the two heavier species, plain physisorption also takes place, causing the sol vation of H-bonded adducts, which are best defined at low coverage. From th e intensity of the nu(OH) mode of the free hydroxyls as a function of press ure, the equilibrium constant for the H-bond formation is calculated in dif ferent ways, according to the role of physisorption. Several correlations a re drawn for Delta nu(OH) and Delta G degrees. The contribution of the subs tituents to the basicity of the double bond as measured by the shift in fre quency is discussed, as well as the role of H-bonding and van der Waals for ces in dictating the energetics of the interaction.