An experimental and theoretical vibrational spectra of isoniazide Ayberk Yilmaz a , Olcay Bolukbasi a , Meric Bakiler b, * a Istanbul University, Science Faculty, Physics Department, Vezneciler, 34118, Istanbul, Turkey b Mimar Sinan Fine Art University, Physics Department, Besiktas, 34347, Istanbul, Turkey Received 28 August 2006; received in revised form 20 December 2006; accepted 21 December 2006 Available online 12 March 2007 Abstract The molecular structure and vibrational spectra of isoniazide (INH) were calculated by the Density Functional Theory (DFT) method using the B3LYP function with the 6-31++G(d,p) and the Z3PolX basis sets. The representation of the hydrogen bonding effect was achieved with the Polarizable Continuum Model (PCM) including the DFT/B3LYP with the 6-31++G(d,p) basis set. Our calculations were performed with the Gaussian98W package program. The scaled wavenumbers, the modified intensities and the total energy distri- butions (TED) of the vibrational modes of INH were calculated by using Scale 2.0 program. SQM results showed that the scaling factors were transferable to the similar molecules. The solid phase FT-IR and FT-Raman spectra of INH have been recorded in the range of 4000–450 and 4000–100 cm 1 , respectively. The calculated wavenumbers were compared with the corresponding experimental values. Ó 2007 Elsevier B.V. All rights reserved. Keywords: DFT calculations; Z3PolX; PCM; SQM; Isoniazide; Vibrational spectra 1. Introduction Tuberculosis is still one of the most infectious diseases in the world. Although in the United States the infection rate has tremendously declined in the last century, the strains of bacteries resistant to antibiotics have been observed very recently, leading to emergence of tuberculosis once again. INH, which has been widely used in the past, is known to act against mycobacterium tuberculosis, and is still the most widely used drug in antituberculous regimens [1]. The pharmacological activity of the biologically active molecules is inherently related to their chemical structures. In this work, we report a description on the molecular geometry, experimental, and theoretical vibrational spectra of isoniazide molecule. Additionally, the hydrogen bonding and the inter-molecular hydrogen bonding effects on vibra- tional spectra of the molecule were investigated. Experimental information on the molecular vibrations was obtained from the FT-IR and FT-Raman spectra mea- sured. In order to determine optimized geometry and a reli- able assignment for the observed vibrational spectra of INH molecule, quantum chemical calculations were per- formed via Density Functional Theory (DFT). We also have performed DFT and DFT with PCM calculations to investigate the intra- and inter-molecular hydrogen bonding effects on the same molecule. 2. Experimental details INH purchased from Sigma was reagent grade and used without further purification. The solid phase FT-IR spec- trum of INH were obtained by a pellet of the powdered sample with KBr. The spectrum were recorded by a Perkin Elmer FT-IR spectrometer in the range of 4000–450 cm 1 , where the resolution was set to 4 cm 1 with 100 scans. The FT-Raman spectrum of the powdered sample were recorded on a Bruker RFS 100/S FT-Raman instrument equipped with Nd-YAG laser by using 1064 nm excitation. The detector was Ge detector cooled with liquid nitrogen. During the process 100 scans were accumulated. Both 0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.02.036 * Corresponding author. Tel.: +90 212 2366936; fax: +90 212 2611121. E-mail address: mbakiler@msu.edu.tr (M. Bakiler). www.elsevier.com/locate/molstruc Available online at www.sciencedirect.com Journal of Molecular Structure 872 (2008) 182–189