Vibrational assignment and structure of 3-(4-methoxyphenyl)pentane-2,4-dione

The intramolecular hydrogen bond, molecular structure and vibrational frequencies of α-paramethoxyphenyl acetylacetone have been investigated by means of high-level density functional theory (DFT) methods with different basis sets. The geometrical parameters results are compared to the experimental structure obtained from X-ray diffraction experiment and with acetylacetone results. The calculated hydrogen bond strength is 17.33 kcal/mol. The O⋯O distance of about 2.450 Å in α-paramethoxyphenyl acetylacetone suggests that the hydrogen bond in this compound is stronger than acetylacetone. This conclusion is well supported by the NMR proton chemical shifts and O-H stretching mode at 2639 cm−1. On the other hand, the results of theoretical calculations show that the paramethoxyphenyl substitution in α position of acetylacetone results in an increase of the conjugation of π electrons in the chelate ring. This result is in good agreement with the Gilli's symmetry coordinates. The topological properties of the electron density contributions for intramolecular hydrogen bond in α-paramethoxyphenyl acetylacetone and acetylacetone have been analyzed in term of the Bader theory of atoms in molecules (AIM).