Adducts of alkali-metal ions with the C≡C triple bond:: an experimental and ab initio study

Acetylenes may behave either as acidic or basic probes, by virtue of the =C-H group, which is acidic, and of the C=C triple bond, which can interact with positively charged centers, such as acidic hydroxyls or metal cations. The interaction of methylacetylene and acetylene with alkali-metal cations, acting as Lewis sites in MCM-41 materials, is studied through combined spectroscopic and ab initio techniques. On the experimental side, MCM-41 materials with low Al content, exchanged with aqueous solutions of alkali-metal nitrates, have been selected as they provide exposed alkali-metal cations with few basic adjacent oxygens. On the computational side, ab initio density functional techniques have been applied to minimal cluster models, consisting one cation ( Li, Na, K, Cs) and one molecule ( either acetylene or methylacetylene). A thorough characterization of the adsorption of acetylenes on Lewis acidic centers is provided, enlightening the presence of two contributions to the interaction, namely that with alkali-cations, through the C=C triple bond, and the one with basic oxygen species near the Lewis acidic sites, through H-bonding.