Spectrochimica Acta Part A 78 (2011) 1467–1475 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Molecular structure, harmonic and anharmonic frequency calculations of 2,4-dichloropyrimidine and 4,6-dichloropyrimidine by HF and density functional methods Usha Rani a , Halil Oturak b , S. Sudha c , N. Sundaraganesan c,∗ a Research and Development Centre, Bharathiar University, Coimbatore 641046, India b Suleyman Demirel University, Department of Physics, Isparta 32260, Turkey c Department of Physics (Engg.), Annamalai University, Annamalai Nagar, Chindambaram, Chidambaram 608 002, Tamil Nadu, India article info Article history: Received 11 December 2010 Received in revised form 31 December 2010 Accepted 17 January 2011 Keywords: Vibrational spectra HF DFT 2,4-DCP 4,6-DCP Natural charges abstract Quantum chemical calculations of energies, geometrical structural parameters, harmonic and anhar- monic frequencies of 2,4-DCP and 4,6-DCP were carried out by HF and density functional theory methods with 6-311++G(d,p) as basis set. The assignment of each normal mode has been made using the observed and calculated frequencies, their IR and Raman intensities. A detailed interpretation of the FT-IR and FT-Raman spectra of 2,4-DCP and 4,6-DCP was reported on the basis of the calculated potential energy dis- tribution (PED). A comparison of theoretically calculated vibrational frequencies at B3LYP/6-311++G(d,p) with FT-IR and FT-Raman experimental data shows good agreement between them. Natural atomic charges of 2,4-DCP and 4,6-DCP were calculated and compared with pyrimidine molecule. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The structural diversity and biological importance of pyrim- idines have made them attractive targets for synthesis over many years. The pyrimidine ring system undoubtedly belongs to the most important heterocycles in nature, as it represent the main structure of many biologically significant compounds, including nucleosides and nucleotides. For this reason many analogs and derivatives of pyrimidines have been synthesized and developed as pharmacologically active compounds or drugs. The pyrimi- dine is a widespread heterocyclic moiety, which is present in numerous natural products as well as synthetic pharmacophores with biological activities [1]. Pyrimidine and condensed pyrimi- dine derivatives exhibit a broad spectrum of biological activities such as anticancer, anti-inflammatory, COX inhibitor, antiallergic and analgesic [2–7]. The realization of the biochemical importance of nucleic acids has occasioned a great revival of interest in the chemistry of pyrimidines. Among them, substituted pyrimidines have become increasingly important as biologically interesting ∗ Corresponding author. Tel.: +91 944 206 8405. E-mail address: sundaraganesan n2003@yahoo.co.in (N. Sundaraganesan). compounds, materials and ligands in molecular recognition [8]. Pyrazine, pyridazine, pyrimidine and their derivatives form an important class of compounds present in several natural flavors and complex organic molecules. Chlorinated pyrimidine substrates can, for example, be utilised in various palladium catalysed cross- coupling reactions [9] and nucleophilic aromatic substitution processes [10,11]. Recently, X-ray analysis and geometrical parameters for 2,4- dichloropyrimidine were obtained by slow evaporation of a methanol solution [12]. To the best of our knowledge, neither quantum chemical calculation, nor the vibrational spectra of 2,4- dichloropyrimidine and 4,6-dichloropyrimidine (hereafter they are, respectively, referred to as 2,4-DCP and 4,6-DCP) have been reported. Therefore, the present investigation was undertaken to study the vibrational spectra of this molecules completely and to identify the various modes with greater wavenumber accuracy. Ab initio HF and density functional theory (DFT) cal- culations have been performed to support our wavenumber assignments. 2. Experimental The samples 2,4-DCP and 4,6-DCP were purchased from Sigma–Aldrich Chemical Company with a stated purity of greater 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.01.028