Structure and vibrational assignment of the enol form of 1-chloro-1,1-difluoro-pentane-2,4-dione

Molecular structure of 1-chloro-1,1-difluoro-pentane-2,4-dione (monochlorodifluoro-acetylacetone, CDFAA) has been investigated by means of ab initio and Density Functional Theory (DFT) calculations and the results were compared with those of 1,1,1-trifluoro-pentane-2,4-dione (trifluoro-acetylacetone, TFAA). The harmonic vibrational frequencies of the two most stable cis-enol forms were calculated at the B3LYP level of theory using 6-31G∗∗ and 6-311++G∗∗ basis sets. We also calculated the anharmonic frequencies at the B3LYP/6-31G∗∗ level of theory for these two stable cis-enol isomers. The calculated frequencies and the Raman and IR intensities were compared with the experimental results. The energy difference between the two stable cis-enol forms, calculated at the MP2/6-31G∗∗ level, is only 3.42 kJ/mol. The observed vibrational frequencies and intensities in the gas phase are in excellent agreement with the corresponding values calculated for the two most stable conformers. However, the observed IR and Raman frequencies also indicate coexisting of both tautomers in the liquid phase and in solution. According to the theoretical calculations, at the B3LYP/6-311++G∗∗ level, the hydrogen bond strength for the most stable conformer is 54.45 kJ/mol, about 2.4 kJ/mol less than that of the corresponding conformer of TFAA. These results are in agreement with the obtained experimental data.