INFRARED-SPECTROSCOPY STUDY OF THE ADSORPTION OF CARBONYL-COMPOUNDS ON SEVERELY OUTGASSED SILICA - SPECTROSCOPIC AND THERMODYNAMIC RESULTS

Fourier transform infrared spectroscopy data are reported concerning t he adsorption of formaldehyde, acetaldehyde, acetone, cyclopentanone, cyclohexanone, and 2-cyclohexen-1-one on Aerosil outgassed at 800 degr ees C. With the four heavier species, plain physisorption competes wit h the formation of H-bonded complexes with the isolated surface hydrox yl, which are best studied at vanishing coverage. The stretching mode of H-bonded hydroxyls shows in all cases an unusually complex shape (c hanging when physisorption takes place), because of the coupling betwe en the OH stretch and the strongly anharmonic intermolecular modes. Fr om the intensity of the nu(OH) mode of the free hydroxyls as a functio n of pressure, the equilibrium constant for the H-bond formation is ca lculated. Several correlations are drawn for Delta nu(OH), Delta nu(CO ), and Delta G degrees. An estimate for Delta H degrees is made from D elta nu(OH), SO that a fairly complete thermodynamic characterization of the H-bonding process is achieved.