Conformation and vibrational spectra and assignment of 2-thenoyltrifluoroacetone

The conformational stabilities of different tautomers of 2-thenoyltrifluoroacetone (4,4,4-trifluoro-1-(2-thienyl)butane-1,3-dione, TTFA) in several solutions have been investigated at the B3LYP level using 6-311++G** basis set. The self-consistent Onsager reaction field model was used to study the solvent effects. It was determined that the energy differences of the most stable conformers (stabilized by the hydrogen bridge), which are negligible in the gas phase, increase in the solvent media. The harmonic vibrational frequencies were calculated at the B3LYP level using 6-31G** and 6-311G** basis sets. A complete vibrational assignment has been clearly provided for the experimental IR and Raman spectra of TTFA and its deuterated analogue, which shows coexisting of four conformations in the sample. This vibrational spectroscopy analyses confirms more content of the “A” forms in the solution than the “B” forms, although the reverse is true in the solid state, in agreement with the theoretical DFT calculations and the experimental X-ray results.