Structure and vibrational assignment of the enol form of 3-chloro-pentane-2,4-dione

Molecular structure of 3-chloro-pentane-2,4-dione (known as α-chloroacetylacetone, ClAA) has been investigated by means of ab initio and Density Functional Theory (DFT) calculations and the results were compared with those of its parent, pentane-2,4-dione (known as acetylacetone, AA). The harmonic vibrational frequencies of the cis-enol form were calculated at the B3LYP level of theory using 6-311G** and 6-311++G** basis sets. We also calculated the anharmonic frequencies at B3LYP/6-31G** level of theory. The calculated frequencies and the Raman and IR intensities were compared with the experimental results.According to the theoretical calculations, at the B3LYP/6-311++G** level, the hydrogen bond strength for ClAA is 68.7 kJ/mol, about 2.3 kJ/mol stronger than that in AA. This result is in agreement with the OH/OD stretching, the OH/OD in-plane bending, and O⋯O stretching frequencies.The 1H chemical shifts for ClAA and AA are computed at the B3LYP/6-311++G** level using the GIAO method. The calculated difference chemical shift of the enolated proton in ClAA and AA is in excellent agreement with the experimental results.Natural bond orbital (NBO) analyses indicate that, in spite of electron withdrawing nature of Cl atom, the steric effect increases the hydrogen bond strength.