Infrared study of the adsorption of N,N-diethylacetamide and three N,N-diethylaminoketones on silica immersed in carbon tetrachloride

Abstract

Infrared spectra are reported of silica immersed in carbon tetrachloride containing an amide and three diethylaminoketones with general formulae Et₂N(CH₂)_nCOCH₃(n= 0, 1, 2 and 3). Adsorptive interactions involved either proton transfer from isolated surface silanol groups to diethylamino groups or the formation of hydrogen bonds between silanol groups and carbonyl or diethylamino groups. Adsorbate molecules each interacted either through one functional group (CO or NEt₂) with single silanol group adsorption sites, or simultaneously through both functional groups with a pair of vicinal isolated silanol groups. Single silanol groups formed hydrogen bonds with the acceptor oxygen atoms in adsorbed N,N-diethylacetamide (n= 0) molecules. For the diethylaminoketones (n= 1, 2 and 3) hydrogen bonding or proton transfer interactions involving the amino group were more favourable than hydrogen bonding interactions involving the carbonyl group. However, increasing the value of n enhanced the formation of surface complexes in which both functional groups were simultaneously liganded to the surface. Modes of adsorption involving a single ligand became more significant as the coverage of adsorption sites was increased. Surface catalysed condensation or fission reactions of N,N-diethylaminoketone adsorbates led to partially characterised desorbed reaction products.