Vibrational assignment and structure of benzoylacetone: A density functional theoretical study

Molecular structure and vibrational frequencies of benzoylacetone (BA) have been investigated by means of density functional theory (DFT) calculations. In addition, the geometry of the cis- and trans-enol conformers were also optimized at MP2/6-31G** level of theory. The results were compared with those of acetylacetone (AA), the parent molecule. IR and Raman spectra of BA and its deuterated analogue were clearly assigned. Comparing the calculated and experimental band frequencies and intensities suggests coexisting of both stable cis enol conformers in comparable proportions in the sample.The energy difference between the two stable chelated enol forms is negligible, about 0.3 and 0.1 kcal/mol, calculated at B3LYP/6-311++G** and MP2/6-31G** levels of theory, respectively. The calculated hydrogen bond energies for these tautomers are about 16 kcal/mol, calculated at B3LYP/6-311++G** level of theory. The molecular stability and the hydrogen bond strength were investigated by applying the NBO and geometry calculations. The theoretical calculations indicate that the hydrogen bond in BA is relatively stronger than that in AA. The averaged geometrical parameters, obtained by superposition of calculated corresponding geometrical parameters for 2BA and 4BA conformers, are in good agreement with the X-ray and neutron diffraction data.