Acquired immunodeficiency syndrome (AIDS, is a disease in which body’s immune system breaks down and is unable to fight off infection caused by human immune deficiency virus (HIV). HIV infects the human cells and uses

Epivir is an anti HIV agent belonging to the class of nucleosides reverse transcriptase inhibitors. These drugs interrupt the virus to make copy of it. A systematic quantum chemical study and vibrational spectra of Epivir has been reported. Structure and spectral characteristic of Epivir have been studied using vibrational spectroscopy and quantum chemical methods. Density function theory calculations of optimized geometry and vibrational spectra have been carried out by Gaussian03, using 6-311G basis set and B3PW91 functional. Based on these results we have discussed the correlation between these vibrational modes and crystalline structure of Epivir. A complete analysis of experimental IR and Raman spectra has been reported on the basis of wave number of the vibrational bands. The IR and the Raman spectra of the molecule based on DFT calculations shows reasonable agreement with the experimental results. The calculated HOMO, LUMO shows that the charge transfer takes place within the molecule. © 2016 Elixir All rights reserved. Elixir Vib. Spec. 100 (2016) 43644-44648 Vibrational Spectroscopy Available online at www.elixirpublishers. com (Elixir International Journal) Yugal Kishor Tiwari and R. A. Singh / Elixir Vib. Spec. 100 (2016) 43644-44648 43645 2. Computational Analysis We mimic the crystalline structure by a small subset of a molecular unit shown in figure1. Complete information about the structural characteristics and vibrational mode of Epivir molecule is obtained by DFT using Gaussian 03 [13, 14] . The vibrational spectrum is obtained at the same level of theory to test the stability of their computed molecular structure. A complete vibrational assignment is also conducted for Epivir, for this purpose the vibrational wave number in the harmonic approximation were calculated which also provide weightage value of internal co-ordinate for vibrational assignment. In order to test the reliability of our modeling techniques we compare bond length and bond angle with the experimental values. Comparisons of experimental and theoretical values of bond length and bond angles are given in table 1 and table 2 respectively. Since DFT vibrational number are known to be higher than experimental wave numbers due to neglect of anharmonicity effects they are scaled down by dual scaling procedure. Hall et al. [15] made a critical analysis of experimentally measured and theoretically calculated wave numbers at same level of theory and divided the normal modes in two regions. The region below 1800 cm -1 called finger print region and the region above 1800 cm -1 include X-H stretching mode (16) . Further as demonstrated in their report dual scaling give the better agreement between theoretical and experimental results. The dual scaling factor 0.9927 and 0.9659 for region below 1800 cm -1 and above 1800 cm -1 regions respectively are used in present study to avoid the systematic error caused by ignoring the anharmonicity and electron density [17,18] . The vibrational assignments of different modes were made by analyzing the result after calculations using Gauss view 4.1 program [19] finally calculated vibrational wave number gives the thermodynamic properties. Fig 1.Ball stick model of the Epivir molecule. Graphical representation of IR and Raman spectra were made by using Gauss view program by combining the results of the Gauss view program and with symmetry consideration vibrational wave number assignment were made with a high degree of accuracy. There are always some ambiguity in defining internal co-ordinates, however in this study the internal co-ordinates define form a complete set and the atomic motion of all the normal modes were observed using the Gauss view program. 3. Experimental details Infrared spectra were recorded on a Shimadzu 8400 S-FTIR spectrometer with a spectral resolution of 4 cm -1 in the region 400-4000 cm -1 . Sample was prepared from mixture of KBr in the ratio 99:1. The FT-Raman spectra were recorded on a RENISHAW INVIA Raman Microscope attach with HeNe laser of wavelength 633nm in the region 400-4000 cm -1 using 10mw power. The sample was measured in a hemispherical bore of an aluminium sample holder. 4. Result and Discussion 4.1. Geometry optimization and energies Since molecular geometry plays an important role in determining the structural activity relationship, the structural analysis provides meaningful information related to the drug action because in the case of flexible molecules, the receptor is likely to alter the solution conformation upon binding. Fig.2 shows the equilibrium state of Epivir. The molecular structure of Epivir belongs to the C1 point group symmetry, the structural parameter of the molecule obtained from geometry optimization are similar to the experimentally available data [20] . The comparison of theoretical and experimental bond length and bond angle is shown in Table.1 and Table.2 respectively. Table 1. Theoretically calculated and experimentally observed bond length. Bond Length B3LYP B3PW91 Experimental * C1C4 1.5144 1.522 1.52 C1O25 1.447 1.407 1.40 C4S23 1.915 1.8620 1.84 C4O6 1.434 1.404 1.40 S23C21 1.874 1.812 1.80 C21C7 1.527 1.529 1.52 C7O6 1.455 1.415 1.40 C7N9 1.455 1.450 1.45 N9C19 1.435 1.436 1.40 C19O20 1.246 1.215 1.21 C19N18 1.369 1.359 1.33 C14N18 1.331 1.314 1.30 C14C12 1.433 1.432 1.40 C12C10 1.357 1.353 1.34 C10N9 1.368 1.356 1.33 C14N15 1.357 1.353 1.35 Table 2.Theoretically calculated and experimental bond