Ucun F, Sa?lam A, Delta E (2015) Ground State Hydrogen Conformations and Vibrational Analysis of Isomers of Dihydroxyanthraquinone by Density Functional Theory Calculation. JSM Chem 3(1): 1015.

Dihydroxyquinones have important applications as a prominent family of pharmaceutically active and biologically relevant chromophores, as an analytical tool for the determination of metals, and in many aspects of electrochemistry [1]. Alizarin, quinizarin, danthron and anthraflavic acid are isomers of dihydroxyquinone. 1,2-Dihydroxyanthraquinone (alizarin) is a red coloring mordant dye, and used as an acid-base indicator in the determination of fluorine. 1,4-Dihydroxyanthraquinone (quinizarin) and 1,8-dihydroxyanthraquinone (danthron) are the simplest molecules showing the chromophore framework peculiar to several compounds of biological and pharmaceutical interest. 2,6-Dihydroxyanthraquinone (anthraflavic acid) is an isomer of the well known alizarin dye and a compound used from commercial suppliers without further purification. Danthron is present in some antitumor drugs. The structure of quinizarin has been subject of numerous spectroscopic investigations, including fluorescence studies in Shpolskii matrices [2], resonance Raman and infrared spectroscopy [3,4], laser spectroscopy in supersonic expansion [5], and X-ray crystallographic investigations [6]. For danthron also, fluorescence studies in Shpolskii matrices [7,8], resonance Raman [9] and infrared spectroscopy [10] studies have been made.

After the development by Lee and co-workers, infrared spectroscopy combined with ab initio quantum theoretical calculations has become a powerful and general method to find the ground state conformations of molecular clusters. An ab initio study of 1,4-, 1,5- and 1,8-dihydroxyanthraquinone was conducted to identify the absolute minimum [11]. Electronic structure of alizarin, two of its isomers, with different transition metal complexes and five rare-earth complexes were studied by using density functional theory (DFT) [12]. Experimental (FT-IR and Raman) and theoretical (B3LYP and B3PW91) vibrational analysis of quinizarin were studied by Xuan and et al [13]. The interaction between quinizarin and metal ions was studied by UV–Visible and fluorescence spectroscopies in solution and, the complex structures were confirmed by time-dependent density functional theory calculations [14]. In the present study we have calculated the optimized molecular geometries and vibrational analysis of isomers of dihydroxyquinone molecule using density functional theory (B3LYP) method with 6-31G (d,p) basis set to find out their ground state hydrogen conformations.

The optimized conformations and vibrational frequencies of dihydroxyquinones have been calculated by using DFT/B3LYP method at 6-31 G (d,p) basis set level. All the computations were performed using Gaussian 03 program package on personal computer [15] and Gauss-View molecular visualization program [16]. The scale factor of 0.9613 was used for B3LYP with 6-31G (d,p) basis set [17]. The proposed vibrational assignments were made by inspection of each of the vibrational mode by GaussView molecular visualization program

Dihydroxyquinones are molecules having 26 atoms, and belong to the point group CS . The three Cartesian displacements of the 26 atoms provide 78 internal modes, namely;

normal modes of vibration. All the vibrations are active both in infrared (IR) and Raman (R). For an N-atomic molecule, 2N-3 of all vibrations is in plane and N-3 is out of plane [18]. Thus, for dihydroxyquinone molecules, 49 of all the 72 vibrations are in plane and 23 out of plane. Since the molecules belong to the CS group all the vibrations being anti-symmetric through the mirror plane of symmetry σ h will belong to the species A′′ and the others being symmetric through σ h belong to the species A′. Thus, since the compounds are planer all the vibrations of the A′ species will be in plane and those of the A′′ species will be out of plane.

The ab initio optimized structures of all the possible hydrogen conformers of the isomers of dihydroxyquinone are illustrated in (Table 1-4). The tables also show the correlation factors for the experimental and calculated geometrical parameters (bond lengths and bond angles) and vibrational frequencies. The experimental vibration values of the compounds are taken from the web page of Rio-Db Spectral Database for Organic Compounds [19], and the experimental parameters form the literature [20- 24]. The correlation graphic at DFT 6-31G (d,p) level for alizarin are drawn in (Figure 1). In (Table 1- 4) are also given the sum of electronic and zero-point energies. As seen from these tables the correlation factors for the conformers with minimum energy of all the isomers are almost best. So, for all the compounds the preferential conformer in the ground state is the conformer with the doubly bonded O atom linked intramolecularly by the two hydrogen bonds. The tables also show the relative energies and the mean vibrational deviations ( ave ν? ) between the calculated

The ground state hydrogen conformations of 1,2- (alizarin), 1,4- (quinizarin), 1,8 -(danthron) and 2,6-(anthraflavic acid) dihydroxyanthraquinone have been investigated using density functional theory (B3LYP) method with 6-31 G (d,p) basis set. The calculations indicate that the compounds in the ground state exist with the doubly bonded O atom linked intra-molecularly by the two hydrogen bonds. The vibrational frequencies and optimized geometry parameters of all the possible conformers of alizarin isomer were given.

•    Dihydroxyquinone
•    Hartree-fock
•    Density functional theory
•    Infrared
•    Vibration