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

The intramolecular hydrogen bond, molecular structure and vibrational frequencies of a-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 (OO)-O-... distance of about 2.450 angstrom in alpha-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 alpha position of acetylacetone results in an increase of the conjugation of pi 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 alpha-paramethoxyphenyl acetylacetone and acetylacetone have been analyzed in term of the Bader theory of atoms in molecules (AIM). (c) 2005 Elsevier B.V. All rights reserved.