Experimental, ab initio and density functional theory studies on sulfadiazine Gulce Ogruc-Ildiz a , Sevim Akyuz a, * , Aysen E. Ozel b a Istanbul Kultur University, Science and Letters Faculty, Physics Department, Atakoy Campus, 34156 Bakirkoy, Istanbul, Turkey b Istanbul University, Science Faculty, Physics Department, Vezneciler 34134, Istanbul, Turkey article info Article history: Received 31 October 2008 Accepted 31 December 2008 Available online 10 January 2009 Keywords: Ab initio HF Density functional theory DFT Infrared and Raman Spectroscopy Sulfadiazine abstract In the present study, combined experimental and computational study on molecular vibrations of free sulfadiazine has been reported. The theoretically possible stable conformers of free sulfadiazine molecule in electronically ground state were searched by means of torsion potential energy surfaces scan studies through C1AC7AS8AN9, C7AS8AN9AC10 and S8AN9AC10AN11 dihedral angles, at both semi-empiri- cal PM3 and B3LYP/3-21G levels of theory. The final geometrical parameters for the obtained stable con- formers were determined by means of geometry optimization carried out at ab initio HF/6-31G++(d,p) and DFT/B3LYP/6-31G++(d,p) theory levels. The harmonic and anharmonic vibrational wavenumbers and IR intensities were calculated at the same theory levels used in geometry optimization. The modes of the fundamental vibrations were characterized depending on their the total energy distribution (TED%). In order to fit the calculated harmonic wavenumbers to experimental ones, dual scale factors were used. The experimental infrared and Raman spectra of sulfadiazine in solid phase have been mea- sured and compared with the calculated vibrational spectra of each conformer. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Sulfadiazine is a member of sulfonamides or sulfa drugs and widely used as antibacterial agents. It eliminates bacteria which cause infections by preventing synthesis of folic acid needed by bacteria for growth [1]. It is used for the treatment of a variety of infections such as toxoplasmosis, urinary tract infections, malaria and a certain type of brain infections [1–3]. Silver and zinc com- plexes of sulfadiazine are used as antibacterial cream for burn infection treatment of animals [4,5]. Despite to its importance in medical and biological fields, up to our knowledge, no study on systematical conformational analysis or theoretical frequency cal- culations has been reported so far for sulfadiazine. Only studies regarding the experimental crystal structure [6] and experimental vibrational spectra [7–9] of sulfadiazine were published. The aims of this study are to determine the possible stable conformers of free sulfadiazine molecule in the room temperature and to calcu- late the vibrational modes and wavenumbers of each conformer in the frameworks of both HF and DFT/B3LYP theory levels employing 6-31G++(d,p) basis set. 2. Methods and calculations 2.1. Experimental Sulfadiazine sample was regeant grade (Aldrich) and used as re- ceived. The IR spectrum of the KBr disc of solid sulfadiazine was re- corded on a Jasco 300 FT-IR spectrometer in the range 400– 4000 cm 1 with 2 cm 1 resolution and 200 scans were accumu- lated. The FT-Raman spectra was recorded by Bruker RFS 100/S instrument using 1064 nm excitation from an Nd:YAG laser. Detec- tor was a liquid nitrogen cooled Ge detector and 1000 scans were accumulated at a resolution of 4 cm 1 . Spectral manipulations such as baseline adjustment, and band fitting were performed using GRAMS/AI Thermo Glactic software. Band fitting was done using a Lorentz function and fitting was undertaken until reproducible results were obtained with squared correlation of r 2  0.9997. 2.2. Computational details The structural model of the free sulfadiazine molecule is given in Fig. 1. All calculations have been performed with the Gaussian03 software package [10]. The possible stable conformers of free sul- fadiazine were determined by single point energy calculations per- formed both semi-emprical PM3 and DFT/B3LYP/3-21G theory levels. In order to find the final geometric parameters of the ob- tained stable conformers, each structure were optimized at DFT/ B3LYP/6-31G++(d,p) theory level. The vibrational modes and corre- sponding wavenumbers and IR intensities of the conformers were calculated using at the HF/ 6-31G++(d,p) and DFT/B3LYP /6- 31G++(d,p) theory levels. The absence of imaginary frequencies confirmed that the stationary points found correspond to real min- ima, instead of being saddle points. The calculated harmonic wave- numbers are scaled by dual scale factors proposed by Balci [11] to fit the experimental ones. Anharmonic corrections to wavenum- bers of the global conformer of sulfadiazine were obtained by 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.12.067 * Corresponding author. Tel.: +90 212 661 9274; fax: +90 212 498 4401. E-mail address: s.akyuz@iku.edu.tr (S. Akyuz). Journal of Molecular Structure 924-926 (2009) 514–522 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc