IR study of the adsorption of unsaturated hydrocarbons on highly outgassed silica: Spectroscopic and thermodynamic results

Fourier transform IR data are reported concerning the adsorption of ethyne, propyne, 1-butyne, 3-hexyne, benzene, and cyclohexene on Aerosil outgassed at 1073 K. With the three lightest acetylenic molecules, an ideal Langmuir-type interaction occurs. With the three six-carbon atom species, physisorption competes with the formation of H-bonded complexes, which are best studied at low coverage. From the intensity of the nu(OH) mode of the free hydroxyls as a function of pressure, the equilibrium constant for 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(o), based on the different basicities of the C-C triple bond as modulated by the alkyl substituents, and on the different C-C bond order in the other molecules: An estimate for Delta H-o is made from Delta nu((OH)) so that a fairly complete thermodynamic characterization of the H-bonding process is achieved. An explanation is proposed for the complex shape of the nu((OH)), band in the case of 3-hexyne. The interplay between sheer H-bonding formation and dispersive contributions to the interaction is also discussed.