Structure and vibrational assignment of the enol form of 1,1,1-trifluoro-2,4-pentanedione

Molecular structure of 1,1,1-trifluoro-pentane-2,4-dione, known as trifluoro-acetylacetone (TFAA), has been investigated by means of Density Functional Theory (DFT) calculations and the results were compared with those of acetylacetone (AA) and hexafluoro-acetylacetone (HFAA). The harmonic vibrational frequencies of both stable cis-enol forms were calculated at B3LYP level of theory using 6-31G** and 6-311++G** basis sets. We also calculated the anharmonic frequencies at B3LYP/6-31G** level of theory for both stable cis-enol isomers. The calculated frequencies, Raman and IR intensities, and depolarization ratios were compared with the experimental results.The energy difference between the two stable cis-enol forms, calculated at B3LYP/6-311++G**, is only 5.89 kJ/mol. The observed vibrational frequencies and Raman and IR intensities are in excellent agreement with the corresponding values calculated for the most stable conformation, 2TFAA. According to the theoretical calculations, the hydrogen bond strength for the most stable conformer is 57 kJ/mol, about 9.5 kJ/mol less than that of AA and about 14.5 kJ/mol more than that of HFAA. These hydrogen bond strengths are consistent with the frequency shifts for OH/OD stretching and OH/OD out-of-plane bending modes upon substitution of CH3 groups with CF3 groups. By comparing the vibrational spectra of both theoretical and experimental data, it was concluded that 2TFAA is the dominant isomer.