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Vibrational Spectroscopy 53 (2010) 112–116

Contents lists available at ScienceDirect

Vibrational Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /v ibspec

uantum chemical and experimental studies on the structure and vibrationalpectra of efavirenz

oni Mishraa, Deepika Chaturvedia, Anubha Srivastavaa, Poonam Tandona,∗, A.P. Ayalab, H.W. Sieslerc

Department of Physics, University of Lucknow, University Road, Lucknow 226007, Uttar Pradesh, IndiaDepartamento de Física, Universidade Federal do Ceará, C.P. 6030, 60.455-900 Fortaleza, CE, BrazilDepartment of Physical Chemistry, University of Duisburg-Essen, Essen D45117, Germany

r t i c l e i n f o

rticle history:eceived 3 September 2009eceived in revised form 7 March 2010ccepted 9 March 2010

a b s t r a c t

Efavirenz, (S)-6-chloro-4(cyclopropylethynyl)-1, 4-dihydro-4-(trifluoromethyl)-2H-3,1-benzoxazin-2-one, is a anti HIV agent belonging to the class of the non-nucleoside inhibitors of the HIV-1 virus reversetranscriptase. Structural and spectral characteristics of efavirenz have been studied by methods of Ramanspectroscopy and quantum chemistry. The optimized geometries, electronic charge distribution, dipole

vailable online 16 March 2010

eywords:favirenzFTF

moments and electrostatic potential surface were calculated by Hartree–Fock and Density functionalB3LYP method with the 6-311++G(d,p) basis sets. Electrostatic potential surfaces have been mapped overthe electron density isosurfaces to obtain information about the size, shape, charge density distributionand chemical reactivity of the molecules.

© 2010 Elsevier B.V. All rights reserved.

ptimized geometriesSP

. Introduction

Efavirenz, (S)-6-chloro-4(cyclopropylethynyl)-1,4-dihydro-4-trifluoromethyl)-2H-3,1-benzoxazin-2-one (Fig. 1), is a anti HIVgent. Efavirenz (EFV) is a non-nucleoside reverse transcriptasenhibitor (NNRTI) used in the treatment of patients with humanmmunodeficiency virus (HIV) infection [1]. NNRTIs are highlypecific and less toxic than nucleoside inhibitors, their thera-eutic effectiveness is limited by relatively rapid emergence ofrug-resistant HIV-1 strains [2]. EFV is used in combination withither protease inhibitors (PIs) or nucleoside reverse transcriptasenhibitors [3,4]. EFV is an oral medication that is used for thereatment of infections with the HIV. It is similar to nevirapine andelavirdine. EFV is not effective against HIV-2, as the pocket of theIV-2 reverse transcriptase has a different structure, which confers

ntrinsic resistance to the NNRTI class [5]. The non-nucleosidenhibitors of the HIV-1 virus reverse transcriptase (RT) (NNRTI)ave recently received a lot of attention [6–9].

Literature survey reveals that to the best of our knowledge,either the complete Raman spectra nor the quantum mechani-al calculations for molecular structure of EFV have been reportedo far. In the present communication, we report detailed quantum

∗ Corresponding author. Tel.: +91 522 2782563; fax: +91 522 2740840.E-mail addresses: poonam tandon@hotmail.com, poonam tandon@yahoo.co.uk

P. Tandon).

924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.vibspec.2010.03.007

chemical studies of the molecular structure and Raman spectra ofEFV using extended basis sets. Information about the geometry andstructure of the molecule and their electrostatic potential surfaces,together with complete analysis of the Raman spectra, based on fre-quency and intensity should help in understanding the structuraland spectral characteristics.

2. Experimental details

The FT Raman spectra were recorded on a Bruker IFS 55EQUINOX with Raman attachment which uses a 1064 nm Nd-YAGlaser line as the excitation line for recording the Raman spectra inthe region of 3400–200 cm−1. The samples were measured in thehemi-spheric bore of an aluminum sample holder. The spectral res-olution of this instrument was also 4 cm−1. FT Raman spectra wereacquired with 512 scans and a laser power of 500 mW at the samplelocation.

3. Computational details

The electronic structure and optimized geometries of themolecule were computed by the Hartree–Fock (HF) and density

functional theory (DFT) method using Gaussian 03 program [10]package employing 6-311++G(d,p) basis sets. We have utilized thegradient corrected DFT [11] with the three-parameter hybrid func-tional (B3) [12] for the exchange part and the Lee–Yang–Parr (LYP)correlation function [13,14], accepted as a cost-effective approach,

S. Mishra et al. / Vibrational Spectroscopy 53 (2010) 112–116 113

faapt

wg

4

4

i3mFiFo[wltbmo

ndilacre(

obtained by Mulliken population analysis. These are given inTable S2 as supplementary material, which also contains the dipolemoments, energy and ionization potentials obtained on the basis ofoptimized molecular geometries. It may be noted that the oxygenatom O5 has large net negative charge −0.2807 and the adjoin-

Fig. 1. Optimized structure of efavirenz.

or the computation of molecular structure, vibrational frequenciesnd energies of optimized structures. The basis set 6-311++G(d,p)ugmented by ‘d’ polarization functions on heavy atoms and ‘p’olarization functions on hydrogen atoms as well as diffuse func-ions for both hydrogen and heavy atoms was used [15,16].

Isoelectronic molecular electrostatic potential surfaces (ESP)ere calculated and plotted for the molecule by the computer pro-

ram GaussView [17] using DFT B3LYP/6-311++G(d,p) basis.

. Results and discussion

.1. Molecular geometry

The optimized structure parameters of EFV calculated by abnitio HF and DFT B3LYP levels with the 6-31++G(d,p) and 6-11++G(d,p) basis sets are listed in Table S1 as supplementaryaterial in accordance with the atom numbering scheme given in

ig. 1. The optimized structure can be compared with other sim-lar systems for which the crystal structures have been solved.or example, the optimized C–CF3 bond lengths by two meth-ds are longer than that found in the similar compound [1.481 Å]18]. For the bond of C–F, the optimized lengths are matched wellith 2-amino-5-chlorobenzotrifluoride, 1.350 Å [18]. The C–C bond

engths of cyclopropyl ring by two methods are slightly longerhan that in nevirapine [19]. Although there are some differenceetween the theoretical values and experimental values, the opti-ized structural parameters can well reproduce the experimental

nes and they are the bases for thereafter discussion.The conformational stability of EFV was investigated by scan-

ing the dihedral angle C15C14C13C8. The potential energyetermined from these calculations is presented in Fig. 2. Two min-

ma are clearly identified in this figure around 93◦ and 145◦. Theow and high energy conformations were labeled as conformers 1nd 2, respectively. Notice that the energy difference between both

onformers is approximately 1.5 kcal/mol, but the potential bar-ier separating these conformers is very low (∼0.4 kcal/mol). Thus,ven at room temperature, the thermal energy of the molecule∼0.6 kcal/mol) will induce the fast transformation of conformer 2

Fig. 2. Potential energy curve of efavirenz for rotation about C13–C14 bond.

in conformer 1. In Fig. 3 both conformers are compared. The confor-mational main difference is related to the orientation of the alkynetail, which is followed by a change in the orientation of the CF3moiety due to the sp3 hybridization of C14. As a consequence, theoxygen of the oxazine ring is above or below the plane defined bythe benzene ring, as observed in Fig. 3.

4.2. Atomic charges and dipole moments

Total atomic charges on the various atoms of molecule were

Fig. 3. Comparison of conformers 1 and 2 (in orange) of efavirenz. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of the article.)

114 S. Mishra et al. / Vibrational Spect

FrBi

iaaHas

ig. 4. Molecular electrostatic potential mapped on the isodensity surface in theange from −6.295E−2(red) to +6.295E−2(blue) for efavirenz calculated at the3LYP/6-311++G(d,p) level of theory. (For interpretation of the references to color

n this figure legend, the reader is referred to the web version of the article.)

ng carbon atom C21 has a large net positive charge 0.4420. Being

ttached to more electronegative carbon atoms, all the hydrogentoms have a net positive charge; in particular, the hydrogen atom30 has large net positive charge 0.3513. The presence of large neg-tive charge on O5 atom and net positive charge on H30 atom mayuggest the presence of an amide synthon which could favour the

Fig. 5. Experimental and calculated (scaled) Raman spectra of the e

roscopy 53 (2010) 112–116

formation of intermolecular interactions in solid forms. Being theamide–amide one of the most frequent homosynthons, it may beproponed that the crystalline structure of EFV could be character-ized by dimers stabilized through this supramolecular interaction.

4.3. Molecular electrostatic potential

Molecular electrostatic potential (ESP) at a point in the spacearound a molecule gives an indication of the net electrostaticeffect produced at that point by the total charge distribution (elec-tron + nuclei) of the molecule and correlates with dipole moments,electronegativity, partial charges and chemical reactivity of themolecules. It provides a visual method to understand the relativepolarity of the molecule. An electron density isosurface mappedwith electrostatic potential surface depicts the size, shape, chargedensity and site of chemical reactivity of the molecules. Suchmapped electrostatic potential surface has been plotted for the EFVin 6-311++G(d, p) basis set using the computer software GaussViewver.3 [17]. Projections of these surfaces along the molecular planeand a perpendicular plane are given in Fig. 4. The different values ofthe electrostatic potential at the surface are represented by differ-ent colors: red represents regions of most negative electrostaticpotential, blue represents regions of most positive electrostaticpotential, and green represents regions of zero potential. Poten-tial increases in the order: red < orange < yellow < green < blue. In allcases, the shape of the electrostatic potential surface is influencedby the structure and charge density distributions in the moleculewith sites close to the oxygen atom, showing regions of most neg-ative electrostatic potential.

HIV-1 reverse transcriptase (RT) is responsible for the conver-sion of single-stranded viral RNA into double stranded DNA priorto integration into the genome of the human host [20]. In caseof enzyme–inhibitor complex, there is strong hydrogen bondingbetween CO of EFV and NH of Lys101. Due to negative charge

on oxygen atom, there is an attractive interaction between CO ofEFV and the positively charged NH3

+ side chain of Lys101. Thesehydrogen bond interactions have not been observed in such HIV-1RT/nevirapine complex [2] and these interactions are responsiblefor the improved inhibitory affinity of second generation NNRTIs

favirenz in the region, 3700–2000 cm−1 and 1900–200 cm−1.

S. Mishra et al. / Vibrational Spectroscopy 53 (2010) 112–116 115

Table 1Selected experimental and computed (conformer 1) vibrational wavenumbers (in cm−1) of efavirenz.

Modes Experimental vibrational bands B3LYP/6-311++G(d,p) HF/6-311++G(d,p)

Unscaled Scaled Scaled

R2[�(NH)] 3314 3630 3447 3665R3[�(CH2)] 3111 3230 3088 3224R3[�(CH2)] 3093 3216 3075 3219R1[�(CH)] 3051 3206 3066 3206�(C C) 2250 2346 2277 2461R2[�(C O)] 1751 1829 1791 1953R3[ı(CH2)] 1472 1502 1478 1607R3[oop(CH)] 1364 1397 1378 1485R1[ı(CH)] 1170 1186 1174 1283�(CF3) 1146 1177 1165 1268�(CF3) 1138 1159 1147 1240R1[�ring] 1097 1111 1099 1196R3[�ring] + oop (C14C20) 928 940 934 1017

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ecreHtisCtoipmmiRt

R1[oop(CH)] 906oop(C14C15) 864R3[�ring] 809R2[oop(CO)] 742

uch as efavirenz compared with first-generation drugs such asevirapine.

.4. Vibrational spectra

The experimental and calculated (scaled) Raman spectra arehown in Fig. 5. Since the DFT vibrational wavenumbers are knowno be higher than the experimental ones, they were scaled downy the wavenumber linear scaling (WLS) procedure of Yoshida etl. [21] by using the expression:

obs = (1.0087 − 0.0000163 �calc.) �calc.

The Raman scattering cross-sections, ∂�j/∂˝, which are propor-ional to the Raman intensities may be calculated from the Ramancattering amplitude and predicted wavenumbers for each normalodes using the relationship [22,23]:

∂�j

∂˝=

(24�4

45

)((�0 − �j)

4

1 − exp[−hc�j/kT

)(h

8�2c�j

)Sj

here Sj and �j are the scattering activities and the predictedavenumbers, respectively of the jth normal mode; �0 is theavenumber of the Raman excitation; and h, c and k are universal

onstants. Starting from the scaled wavenumbers and theoreticalntensities, the infrared and Raman spectra of the molecule werealculated using Lorentzian band profile (FWHM = 8 cm−1) and ourwn software.

Comparison of the frequencies calculated at HF and B3LYP withxperimental values (Table 1) reveals the over estimation of thealculated vibrational modes due to neglect of anharmonicity ineal system. Inclusion of electron correlation in DFT to a certainxtent makes the frequency values smaller in comparison with theF frequency data. The CH and NH stretching modes are expected

o be observed at the high wavenumber region. The �(CH) stretch-ng bands are observed between 3093 and 3000 cm−1. The verytrong band at 2250 cm−1 may be assigned to the stretch mode of

C bonds. The carbonyl stretching mode �(C = O) corresponds tohe band at 1791 cm−1. Let us start considering the contributionf the cyclopropyl ring (R3), whose �(CH2) stretches are presentn the Raman spectra at 3093 cm−1. The ı(CH2) deformation areresent in the Raman spectra at 1430 cm−1. The CF3 stretching

odes have been assigned at 1138 and 1146 cm−1. The rockingodes of CF3 appear to have variable magnitudes in CF3 contain-

ng benzene. The CF3 rocking mode is observed at 285 cm−1 in FTaman spectra. Notice that the most evident discrepancies betweenhe experimental and calculated spectra are associated with the

905 899 1001872 867 943822 818 884749 746 825

stretching modes of the amide synthon, showing its participationin hydrogen bonds. This observation supports the assumption thatthe amide synthon plays very important role in stabilizing the crys-talline structure of EFV. List of selected observed and calculatedbands of EFV is presented in Table 1.

5. Conclusions

In this study, we computed vibrational spectra of EFV using HFand DFT method with different basis sets. Any discrepancy notedbetween the observed and the calculated frequencies may be dueto the fact that the calculations have been actually done on a singlemolecule in the gaseous state contrary to the experimental valuesrecorded in the presence of intermolecular interactions. Moleculargeometry parameters calculated by DFT agree with the experimen-tal values of similar molecule. Information about the size, shape,charge density distribution and site of chemical reactivity of themolecules has been obtained by mapping electron density isosur-face with ESP. Raman spectra were recorded and the vibrationalbands were assigned on the basis of the DFT calculations. We haveconcentrated in the conformational stability and the descriptionof the vibrational spectra, which are valuable information for thequality control of medicines including EFV.

Acknowledgements

The financial support from the CNPq and DST under Indo-Brazilproject and from Alexander von Humboldt Foundation, Germany isgratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.vibspec.2010.03.007.

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