FT-IR, FT-Raman spectra, density functional computations of the vibrational spectra and molecular geometry of 2-hydroxyquinoxaline S ßenay Yurdakul * , Turgay Polat Gazi Üniversitesi, Fen Edebiyat Fakültesi, Fizik Bölümü, 06500 Teknikokullar, Ankara, Turkey article info Article history: Received 3 July 2009 Received in revised form 21 October 2009 Accepted 21 October 2009 Available online 27 October 2009 Keywords: 2-Hydroxyquinoxaline Infrared spectra Raman spectra Tautomerism Dimer form DFT abstract With the purpose of finding predominant tautomer among seven possible isomers of 2-hydroxyquinox- aline 2-OHQ, its equilibrium geometry and harmonic wavenumbers were calculated by the B3LYP/6- 311++G ** DFT method. The infrared and Raman spectra of 2-OHQ were recorded in the regions 400– 4000 cm À1 , and 50–3500 cm À1 , respectively. Vibrational wavenumbers were compared with IR, and Ra experimental data. Computational calculations at B3LYP/6-31+G ** , and 6-311G ** levels were also employed in the study of the predominant tautomeric form of 2-OHQ. To support our relative energy results we also used the QCISD method. In agreement with experimental results, our calculations show that the keto form of 2-OHQ is the most stable tautomer and the best results were obtained at B3LYP level using 6-311++G ** basis set. All the tautomeric forms of 2-OHQ were determined and optimized. Bond dis- tances, bond angles and dihedral angles are presented. For two most stable tautomers, the calculated wavenumbers were given. In addition, two stable dimer forms of 2-hydroxyquinoxaline calculated at the B3LYP/6-31G * level. The dimer A is by 15.57 kcal mol À1 more stable than dimer B. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction 2-Hydroxyquinoxaline [2-OHQ] is the main metabolite of qui- nalphos [0,0-diethyl-0-(2-quinoxalinyl)-phosphorothioate [1,2]; quinalphos a widely used insecticide and acaricide, is a source of toxicity to humans and vertebrate animals [3]. Substituted quinoxalines are an important class of benzohetero- cycles [4] which constitute the building blocks of some organic semiconductors [5,6] and a wide range of pharmacologically active compounds having anticancer [7], antibacterial [8], and antitu- mour [9–12] activities. Some quinoxaline derivatives serve as DNA photo-cleaver [13]. The quinoxaline moiety also, present in peptide antibiotics such as echinomycin and triostin A [14] and TANDEM is known to inter- calate bifunctionally into DNA [15]. Hence it is of interest to obtain accurate structural parameters of the quinoxaline moiety and its chemical modifications. Only few articles were found in the literature dealt with the determination and calculation of 2-OHQ geometry, and about bond lengths and bond angles. The detailed crystal data on 2-OHQ were reported by Padmaja et al. [16]. They showed that the compound exists in the keto form. In 2000 Ribeiro da Silva et al. reported ther- mochemical studies and theoretical geometric parameters of 2- OHQ in the hydroxy-form (enol form) in the gaseous phase [17]. They also reported only bond distances the enol form of 2-OHQ using the density functional theory and the B3LYP hybrid ex- change–correlation energy functional with 6-311G ** basis set. In 2007 Gerega et al. reported oxo-hydroxy tautomerism and photot- automerism of 2-quinoxalinone molecule by using the matrix-iso- lation technique [18]. They also observed the ratios of population of the oxo and hydroxy tautomers in Ar matrixes correspond to the tautomeric equilibria of the compounds in the gas phase. The relative energies of the DNA-base tautomers were studied by a variety of quantum chemical methods [19], among them density functional theory (DFT) , the ab initio Hartree–Fock (HF) methods, standard second-order Moller–Plesset perturbation theory (MP2), an improved version of it (SCS-MP2), coupled-cluster (CCSD) and the very similar quadratic configuration interaction including sin- gle and double excitations (QCISD), and perturbative corrections for triple excitations [QCISD(T)]. Density functional theory (DFT) calculations are reported to provide excellent vibrational frequencies of organic compounds if the calculated frequencies are scaled to compensate for the approximate treatment of electron correlation, for basis set defi- ciencies and for the anharmonicity [20–26]. Till now a detailed analysis of vibrational frequencies and IR and Raman spectra of 2-hydroxyquinoxaline has not been pub- lished. The aim of this study is to give optimal molecular geometry, vibrational modes of free 2-OHQ and finding predominant tauto- mer among seven possible isomers of 2-OHQ. In this study, we re- port DFT/B3LYP calculation results on 2-hydroxyquinoxaline. We 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.10.035 * Corresponding author. Tel.: +90 3122021238; fax: +90 3122122279. E-mail address: senayy@gazi.edu.tr (S ß. Yurdakul). Journal of Molecular Structure 963 (2010) 194–201 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc