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. ing 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. 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. l bond orbital (NBO) analyses indicate that, in spite of electron withdrawing nature of Cl atom, the steric effect increases the hydrogen bond strength.