Contribution of pyrone-type structures to carbon basicity: An ab initio study

The contribution to carbon basicity of pyrone-type structures is examined i n this work by carrying out ab initio calculations on various cluster model s. The different basic sites in a bicyclic pyrone structure are studied at the MP2/6-311+G(2d,2p)//MP2/6-31G(d) level of theory, rendering a reaction energy of -84.2 kcal/mol when a proton transfer takes place from H3O+ to th e carbonylic oxygen in pyrones. The interpretation of the theoretical resul ts confirms that resonance stabilization is a crucial factor controlling th e basic character of pyrone-type structures. Various effects,like the modif ication of the etheric position, the relative position of non-neighboring o xygen atoms, and the presence of an adjacent basal plane, are also taken in to account. Due to the increase of resonance stabilization observed in larg e cluster models, a broad spectrum of base strength covered by pyrone-type structures is predicted. The reaction energies of these pyrone-type structu res with H3O+ are a few kilocalories per mole more exoergic than that of qu inoline, an illustrative organic base. These results support the outstandin g role played by pyrone-type structures in carbon basicity.