H bonding of substituted pyridines on silica

The infrared spectra of a series of substituted pyridines which were adsorbed on silica have been obtained. A linear correlation is obtained between the wavenumber shift of the OH stretching fundamental of the surface hydroxyl groups towards lower wavenumber and the Hammett polar substituent parameter of the adsorbate. Deviations are only observed for tert.-butyl substituents in ortho-position and for ortho-substituents which render a dual interaction with neighbouring OH groups possible. The optical density of an IR continuous absorption is a function of the proton affinity of the adsorbate and it can be explained by assuming the formation of protonated adsorbed species which exhibit symmetrical NH⁺… N bond.