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  Hindawi Publishing CorporationSpectroscopy: An International JournalVolume 27 (2012), Issue 3, Pages 185–206doi:10.1155/2012/614710 Electronic Structure, Nonlinear OpticalProperties, and Vibrational Analysis of Gemifloxacin by Density Functional Theory Shamoon Ahmad Siddiqui, 1 Tabish Rasheed, 2 Mohd Faisal, 1 Anoop Kumar Pandey, 3 and Sher Bahadar Khan 4 1 Centre for Advanced Materials and Nanoengineering, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia 2  Department of Applied Sciences, School of Engineering and Technology, Sharda University,Plot No. 32–34, Knowledge Park III, Greater Noida 201306, India 3  Department of Physics, University of Lucknow, Lucknow 226001, India 4 Center of Excellence for Advanced Materials Research and Chemistry Department, Faculty of Science,King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia Correspondence should be addressed to Shamoon Ahmad Siddiqui,shamoonasiddiqui@gmail.comCopyright © 2012 Shamoon Ahmad Siddiqui et al. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the srcinal work is properly cited. Abstract. The non-linear optical properties of gemifloxacin (C 18 H 20 FN 5 O 4 ) have been examined using density functionaltheory (DFT). The molecular HOMO, LUMO composition, their respective energy gaps, MESP contours/surfaces have alsobeen drawn to explain the activity of gemifloxacin. The equilibrium geometries and harmonic frequencies of title moleculewas determined and analyzed at DFT/B3LYP level employing the 6-31G(d,p) basis set. The skeleton of both the optimizedmolecules is non-planar. In general, a good agreement between experimental and calculated normal modes of vibrations hasbeen observed.Keywords: Nonlinear optical properties, polarizability, first static hyperpolarizability, MESP, vibrational spectra 1. Introduction Gemifloxacin (7-[(4E)-3-(aminomethyl)-4-methoxyiminopyrrolidin-1-yl]-1-cyclopropyl-6-fluoro-4-oxo-1,8-naphthyridine-3-carboxylic acid) is an oral broad-spectrum quinolone antibacterial agent widelyused in the treatment of acute bacterial exacerbation of chronic bronchitis and mild-to-moderatepneumonia [1–3]. Gemifloxacin acts by inhibiting DNA synthesis through the inhibition of both DNA gyrase and topoisomerase IV enzymes, which are essential for bacterial growth. Notably this drug hasabout 100 times higher affinity for bacterial DNA gyrase than for mammalian ones. Gemifloxacin is abroad-spectrum antibiotic that is highly active against both Gram-positive and Gram-negative bacteria  186 Spectroscopy: An International Journal Figure 1: Molecular structure and numbering scheme of gemifloxacin.[4,5]. Gemifloxacin is globally used for the treatment of bacterial infection caused by susceptible strains like S. pneumoniae, H. influenzae, H. parainfluenzae, or M. catarrhalis, S. pneumoniae (including multi-drug-resistant strains (MDRSP)), M. pneumoniae, C. pneumonia; K. pneumoniae . Gemifloxacin rapidlyabsorbed from the gastrointestinal tract and the absolute bioavailability averages approximately 71 % .Gemifloxacin is metabolized to a certain extent by the liver. All metabolites formed are minor ( < 10 % of the administered oral dose); the principal ones are N-acetyl gemifloxacin, the E-isomer of gemifloxacinand the carbamyl glucuronide of gemifloxacin [6,7]. The aim of the present communication is to investigate the molecular structure, vibrationalspectra, and energetic data analysis of the molecule under study, in gas phase, due to biologicaland pharmaceutical importance of the title molecule. The structure and the ground-state energy of the drug under investigation have been analyzed employing density functional theory with B3LYPmethod. In order to obtain a more complete description of molecular vibration, vibrational frequencycalculation has been carried out. The vibrational analysis also provides the detailed information aboutthe intramolecular vibrations in the fingerprint region. The reported optimized geometries, molecularproperties such as equilibrium energy, HOMO-LUMO gap, dipole moment, polarizability as well asfirst static hyperpolarizability components along with the electrostatic potential contours and surfaceshave also been used to understand the activity of the molecules. 2. Experimental: Structure and Spectra The fourier transform infrared spectrum was recorded with FT-IR Perkin Elmer spectrometer in KBrdispersion in the range of 400 to 4000cm − 1 . The optical properties of the gemifloxacin were examinedusing UV-visible spectrophotometer at room temperature. UV-visible spectrum was recorded in therange of 190–800nm with Perkin Elmer-Lambda 950-UV-visible spectrometer. To measure the UV-visible absorption, the gemifloxacin particles were dispersed in distilled DI water and measured. Themodel molecular structure of gemifloxacin has been given inFigure 1. The experimental and calculatedFT-IR spectra is given inFigure 2, and the experimental UV-visible spectrum is given inFigure 3.  Spectroscopy: An International Journal 187    R  e   l  a   t   i      v   e   I   R   i  n   t  e  n  s   i   t      y 10.504000 3500 3000 2500 2000 1500 1000 500 W a v en u mbers (cm − 1 )(a)104000 3500 3000 2500 2000 1500 1000 5000.5    R  e   l  a   t   i      v   e   I   R   i  n   t  e  n  s   i   t      y W a v en u mbers (cm − 1 )(b) Figure 2: Comparison of normalized IR spectra: (a) experimental (FTIR) and (b) scaled simulatedspectrum obtained by using DFT (scaling factor × 0.96) harmonic calculations for gemifloxacin.    A   b  s  o  r   b  a  n  c  e (nm)2.521.510.50200 300 400 500 Figure 3: UV-visible spectrum of gemifloxacin. 3. Computational Details In the present communication the density functional theory (DFT) [8] has been employed using Becke’sthree-parameter hybrid exchange functionals [9] with the Lee-Yang-Parr correlation functionals [10, 11] to optimize the molecular structure and to calculate the electronic structure properties of the drugmolecule. The Gaussian 03W program [12] was used to calculate the vibrational spectra, dipole moment( μ ), polarizability ( α ), and the first static hyperpolarizability ( β  ) of the title molecule, based on the finitefield approach. The vibrational frequencies are calculated and scaled down by the appropriate factor [13,14]. The vibrational wavenumber assignments and PED calculation have been carried out by combiningthe result of the GaussView 4.1 and the VEDA program [15,16] with symmetry considerations.  188 Spectroscopy: An International Journal    T  o   t  a   l  e  n  e  r      g     y     (   H  a  r   t  r  e  e   ) Scan coordinate200180160140120100806040 − 1372.3860 − 1372.3865 − 1372.3870 − 1372.3875 − 1372.3880 − 1372.3885 Figure 4: PES scan for dihedral angle N 9 –O 3 –C 28 –H 46 at B3LYP/6-31G(d,p).The comparative experimental and calculated FTIR spectrum plotted using the pure Lorentzianband shape is shown inFigure 2. 4. Result and Discussion 4.1. Analysis of Conformers of Gemifloxacin Theoretical calculations for conformers of gemifloxacin were carried out using the B3LYP/6-31G(d,p)method. The plots of the potential energy surface (PES) scans for this molecule are shown in Figures4and5. The dihedral angles N 9 –O 3 –C 28 –H 46 and C 18 –C 23 –C 27 –O 5 are the relevant coordinates forconformational calculations within the molecule. In these calculations, all the geometrical parameterswere simultaneously relaxed during the calculations while the dihedral angles were varied insteps of 10 ◦ , 20 ◦ , 30 ◦ ,..., 360 ◦ . The global minimum energy structure was obtained at − 179.821 ◦ and − 132.892 ◦ for the dihedral angles N 9 –O 3 –C 28 –H 46 and C 18 –C 23 –C 27 –O 5 , respectively. Thecorresponding minimum energy for both PES scans was − 1372.3884 Hartree, which implies thatthe obtained structure was a global minimum. This structure is shown inFigure 1and was used forperforming frequency calculations. 4.2. Molecular Geometry Optimization The equilibrium geometry optimization of lowest energy conformer has been achieved by energyminimization. The optimized geometry of the molecule under study is confirmed to be located at theglobal minima on PES, as the calculated vibrational spectrum contains no imaginary wavenumber. Thegiven molecule has three rings. Out of these two are six membered and one five membered. Ring R1and R2 are in a plane while ring R3 deviates from the given plane due to two bulky groups, one attachedat 6N of ring R1 and the other attached at 19C. The optimized bond length of C–C in six-memberedpyridine ring R1 ranges between 1.367˚A and 1.475˚A, while, for another pyridine ring R2, this ranges
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