2016. 57, 1 UDC 548.737:543.422 DENSITY FUNCTIONAL EFFICIENCY IN THE CALCULATIONS OF VIBRATIONAL FREQUENCIES AND MOLECULAR STRUCTURES OF i-DIKETONES H. Azizi-Toupkanloo, S.F. Tayyari Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran E-mail: Ho_azizi60@yahoo.com Received December, 27, 2014 Revised April, 8, 2015 Density functional theory (DFT) levels are employed to calculate the vibrational frequencies and geometrical data of i-diketones. <...> These types of hydrogen bonds exist in various organic compounds and biomolecules, such as hormones, coenzymes, proteins, and i-diketones (i-dicarbonyl compounds). <...> Among these compounds, i-diketones are the simplest molecules involved in O—H…O intramolecular hydrogen bonds. <...> Therefore, in order to predict their physical properties or even choose one suitable for a specific application, it is necessary to make a link between the fundamental properties of the system, such as molecular interactions, electronic structure, and chemical properties of i-diketones. <...> Different parameters, such as the conformational and structural properties of i-diketones, the possibility of diketo-enol tautomerization, and the nature of the strong intramolecular O—H O hydrogen bond in the enol form were considered by scientists because the enol form of i-diketones is stabilized by a strong intramolecular hydrogen bond (Fig. 1). <...> The resonance conjugation of -electrons is enhanced due to the formation of the hydrogen bond, which causes a marked tendency for the equalization of the bond orders of valence bonds in the resulting six-membered chelate ring [ 13, 14 ]. <...> Structures of enolic acetylacetone theoretical and experimental point of view, the mobility of the hydrogen atom in these compounds determines a considerable number of interesting properties of these compounds. <...> For instance, the vibrational spectra of these compounds have been the subject of several investigations which support the existence of a strong intramolecular hydrogen bond in the enol form of i-dicarbonyl compounds [ 9, 14, 15 ]. <...> Acetylacetone (CH3C(O)CH2C(O)CH3), one of the simple members of this class of compounds, has been studied both experimentally and theoretically to find that it is asymmetrical in its most stable conformation [ 7—9, 16 ]. <...> Acetylacetone was observed in keto and enol forms of tautomers (Fig. 2). <...> As this figure shows, the population of the enolic form of acetylacetone, which exists as a six-membered hydrogen-bonded ring <...>