Experimental (FT-IR) and theoretical (DFT-IR) studies of keto-enol tautomerism in pyruvic acid

FT-IR spectra were recorded for α-keto pyruvic acid in three solvents (CCl4, CH2Cl2, and benzene) at room temperature. In parallel, quantum-chemical calculations were performed for all stable tautomers-rotamers: three stable keto and six stable enol structures using the DFT(B3LYP) method and various basis sets 6-31++G** and/or 6-311++G(3df, 3pd). Comparison of the experimental FT-IR and the computed DFT-IR spectra indicates that pyruvic acid may exist as a mixture of at least four isomers, three keto (Tce, Tte and Cte) and one enol (E1), among which the intramolecularly H-bonded keto Tce structure highly predominates. Intensity of the ν(OH) band assigned to the enol form (E1) varies when proceeding from CCl4 to other solvents. It increases in CH2Cl2 (weak H-bond donor solvent) and decreases in benzene (weak H-bond acceptor solvent) in comparison to the ν(OH) band assigned to the most stable keto form (Tce). Independently on solvent properties, in each case the keto Tce structure is favoured.