Ab initio and Density Functional Calculations on the Ring-Chain Tautomerism of γ-Oxocarboxylic Acids

The results of ab initio (HF, MP2) and density functional (B3LYP) calculations on a series of γ-oxocarboxylic acids [(Z)-3-acetyl or benzoylacrylic and 2-acetyl or benzoylbenzoic acids] A and their cyclic tautomers B are presented. For the open-chain tautomers, different rotamers were considered. Bulk solvent effects were considered by using the polarised continuum model (PCM) and were found to be essential for a proper description of the ring-chain tautomeric equilibrium. Acetyl derivatives generally prefer the cyclic form, whereas for benzoyl derivatives the preference depends on the nature of the link. Introduction of substituents (methyl, phenyl) at C2 and/or C3 for acrylic acid derivatives substantially shifts the equilibrium towards the cyclic form B.