Structure and vibrational assignment of 3,4-diacetyl-2,5-hexanedione. A density functional theoretical study

Fourier transform infrared and Fourier transform Raman spectra of 3,4-diacetyl-2,5-hexanedione, known as tetraacetylethane (TAE) or alpha,alpha’-bis-acetylacetone, and its deuterated analogue have been obtained. Density functional theory (DFT) B3LYP and BLYP calculations have been carried out with the purpose of understanding the vibrational spectra of this compound and its deuterated analogue. The calculated geometrical parameters show a very strong hydrogen bond, compared with its parent molecule, acetylacetone (AA), with an (OO)-O-... distance of 2.464-2.505 angstrom. This bond length is about 0.05-0.06 angstrom shorter than that for AA. According to the theoretical calculations, TAE has an asymmetric structure with a hydrogen bond strength of about 17.3 kcal/mol per bond (calculated with 6311 ++G* * basis set), about 1.4 kcal/mol more than that for AA. This increase in the hydrogen bond strength is consistent with the frequency shifts for OH/OD stretching, OH/OD out-of-plane bending, and (OO)-O-... stretching modes and downfield proton chemical shift upon substitution of alpha-H atom with acetylacetone radical. The geometries of keto-keto and some of the enol-keto tautomers were also fully optimized and compared with the enol-enol tautomer. To investigate the effect of acetylacetone radical on the hydrogen bond strength, the charge distributions, steric effects, and Wiberg bond orders in TAE and AA were studied by the Natural Bond Orbital (NBO) method for optimized model compounds at B3LYP/6-31G** level of theory. The results of NBO analysis indicate that the steric effect is the main factor for increasing the hydrogen bond strength in TAE compared with that in AA. (c) 2006 Elsevier B.V. All rights reserved.