научная статья по теме CONFORMATIONAL ANALYSIS AND VIBRATIONAL SPECTROSCOPIC STUDIES ON DAPSONE Физика

Текст научной статьи на тему «CONFORMATIONAL ANALYSIS AND VIBRATIONAL SPECTROSCOPIC STUDIES ON DAPSONE»

СПЕКТРОСКОПИЯ КОНДЕНСИРОВАННОГО СОСТОЯНИЯ

УДК 539.194

CONFORMATIONAL ANALYSIS AND VIBRATIONAL SPECTROSCOPIC STUDIES ON DAPSONE © 2012 г. Gulce Ogruc Ildiz*, Sevim Akyuz

Istanbul Kultur University, Science and Letters Faculty, Physics Department, Atakoy Campus,

34156 Bakirkoy, Istanbul, Turkey E-mail: g.ogruc@iku.edu.tr Received January 19, 2012

In this study, the theoretical conformation analysis of free dapsone has been performed by single point energy calculations at both semi-empirical PM3 and DFT/B3LYP-3-21G theory levels and three stable conformers were determined. Both the IR and Raman spectra of the molecule in solid phase have been recorded. The IR intensities and harmonic vibrational wavenumbers of each conformer were calculated by DFT method at B3LYP/6-31++G(d, p) theory level. For the fundamental characterization, the total energy distribution (TED) calculations of the vibrational modes were done using parallel quantum mechanic solution program (SQM) and the fundamental modes were assigned. The theoretical results are in agreement with the experimental ones.

INTRODUCTION

Dapsone is a very effective agent against Mycobacterium laprea which causes leprosy [1—3]. It is also used for the treatment of malaria and infections in AIDS patients such as toxoplasmosis, tuberculosis and PCP (Pneumocystic carinii pneumonia) [1, 3]. Besides the antibacterial and antibiotic effects of dapsone, it is also effective against a number of inflammatory diseases such as dermatitis, acne and Behcet's disease [3]. In literature, there are some published experimental and computational studies on dapsone. X-ray crystal structure analysis was done by Bocelli and Dickinson et al. [4, 5] and experimental IR and Raman spectra have been studied by D'Cunha et al. [6]. Raman and SERS spectra of dapsone have been recorded and Raman bands were calculated by using DFT theory employing 6-31G(d) basis set. DFT anaylsis of molecular structure of diphenylsulfone and dapsone have been studied by Forner et al. [7]. In a very recent study, conformational analysis of dapsone was done and only the structure of two low energy isomers were determined [8]. In this study, three stable conformers of dapsone at room temperature were determined by conformational analysis at both semi-empirical PM3 and DFT/B3LYP/3-21G theory levels. The vibrational wavenumbers of each conformer are calculated at DFT/B3LYP/6-31G++(d,p) theory level and compared with the experimental ones. The modes corresponding to the vibrational wavenumbers were characterized by total energy distribution calculations. The effect of conformational changes on molecular structure and vibrational modes was discussed.

MATERIALS AND METHOD

Experimental

The IR spectrum of the KBr disc of solid dapsone was recorded on a Jasco 300 FT-IR spectrometer in the range of 400—4000 cm-1 with 2 cm-1 resolution and 200 scans were accumulated. The FT-Raman spectra of the molecule was recorded by Bruker RFS 100/S instrument using 1064 nm excitation from an Nd:YAG laser. Detector is liquid nitrogen cooled Ge detector. 1000 scans were accumulated at a resolution of 4 cm-1.

Computational Details

The conformational analysis of free dapsone molecule was carried out by single point energy calculations at both semi-empirical PM3 and DFT/B3LYP/3-21G theory levels by changing the dihedral angles around C-N and C-S bonds by 30° in each iteration and all possible stable conformers were determined. The calculations were done by Gaussian03 software package [9]. The molecular model of free dapsone is given in Fig. 1. The optimized geometries, vibrational frequencies, IR intensities of each stable conformer were calculated at DFT/B3LYP/6-31G++(d, p) theory level. The calculated harmonic wavenumbers are scaled by dual scale factors to fit the experimental ones. The harmonic vibrational modes were characterized by total energy distribution (TED %) calculations using the scaled quantum mechanics (SQM) method via parallel quantum solutions (PQS) program [10].

13H-10N-3

29H-27N-24C-20C]

18C-17C-12S-11C

Fig. 1. Molecular model of free dapsone molecule and dihedral angels.

RESULTS AND DISCUSSION

Conformational Analysis

In order to find the stable conformers, conformational analysis was carried out for the title compound. The findings from the calculations show that free dapsone molecule has three stable conformers at room temperature (see Fig. 2). The profile of the potential energy surface for torsions is given in Fig. 3. There were no imaginary frequencies in the obtained calculations. These findings indicate that all obtained stationary points correspond to real minima. The self consistent field energies and zero point corrected relative energy differences of these most stable conformers are given in Table 1. The energy difference between the most stable conformer and the other two conformers is 0.096009 kcal/mol and 0.2133534 kcal/mol, respectively, which is lower than kT energy (0.6 kcal/mol at room temperature).This indicates that all three structures are conformers of dapsone at room temperature.

Molecular Geometry

The calculated geometric parameters of the three stable conformers are given in Table 2 in accordance with the atom numbering scheme given in Fig. 2. The geometric data of each conformer are almost the same

except the dihedral angles around 3C-10N and 24C-27N bonds (see Table 2).The geometry differentiation of each conformer is the orientation of amino groups with respect to the phenyl ring. The comparison of calculated geometric parameters with the crystallo-graphic study of dapsone single crystal [4] shows that the calculated parameters of dapsone by HF theory level fit the bond lengths better than DFT theory level. Despite that, bond angle calculations at DFT theory level gave closer results to experimental ones rather than HF calculations (see Table 2).

Vibrational Assignments

The dapsone molecule has 29 atoms, which possess 81 normal modes of vibrations. The vibrational wave-numbers of the global conformer (I) of dapsone calculated at B3LYP/6-31++G(J, p) theory level, dually scaled (0.955 and 0.977) harmonic wavenumbers, experimentally obtained wavenumbers in solid phase [6] and TED of the modes are given in Table 3. The vibrational wavenumbers of the second (II) and third (III) energetically preferable conformers of title compound calculated at DFT/B3LYP/6-31++G(J, p) theory level and scaled harmonic wavenumbers by dual scale factors (0.955 and 0.977) were given together with the modes in Table 4. Modes were assigned by total energy distribution (TED %) calculations. The vibrational

Table 1. The SCF energies of three conformers of free dapsone molecule

Conformers Calculated SCF energies 1 Relative energy differences3

( Hartree) (Hartree)2 (Hartree) (kcal/mol)

Conf. I -1122.641416 -1122.417175 0.00 0.00

Conf. II -1122.641233 -1122.417022 0.000153 0.096009

Conf. III -1122.640981 -1122.416835 0.000340 0.2133534

1 Calculated at DFT/6-31G++(<i, p) theory level.

2 Zero point vibrational energy corrected.

3 The relative energies of the conformers are given respect to the global conformer (conf. I).

Fig. 2. The three stable conformers of free dapsone molecule; a, b, c alternately correspond to the first (I), second (II) and third (III) stable conformers.

Energy, kcal/mol

Fig. 3. Profile of the potential energy surface for torsions around 11C-12S, 10N-3C bonds.

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