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Autori: V.Chis, A.Parnau, T.Jurca, M.Vasilescu, S.Simon, O.Cozar, L.David
Editorial: Elsevier,Chemical Physics, 316 , p.153-163,, 2005.
Pyrazinamide (PZA), an analogue of nicotinamide, is a very important drug used to treat tuberculosis . Also, some transition metal(II) molecular complexes of this molecule are recognized and used as antimycobacterial agents .
For a proper characterization of these complexes, a reliable assignment of vibrational, NMR and EPR spectra is essential for molecular complexes themselves and for the parent molecule, as well. For this purpose, theoretical methods based on density functional theory (DFT) are invaluable tools. Although some experimental studies are reported on PZA and its complexes [2-5], however none of these is correlated with theoretical results.
In this work we report a joint experimental and theoretical investigation of PZA. FT-IR, Raman and NMR spectra of normal and deuterated PZA have been recorded and analyzed in order to get new insights into molecular structure and properties of this molecule, with particular emphasize on its intra- and intermolecular hydrogen bonds (HB’s). Theoretical calculations were performed by using B3LYP and BLYP exchange-correlation functionals, in conjunction with 6-31G(d) and 6-311G(2df,p) basis sets. A value of 8.33Kcal/mol was estimated for the intramolecular H bond energy, at B3LYP/6-31G(d) level of theory. Intermolecular interactions effects on molecular properties have been considered by calculations performed on PZA dimer. The calculated geometrical parameters (i.e. bond length: 2.895Å and bond angle: 178.4o) of the intermolecular HB are in excellent agreement with the experimental ones and suggest a moderate, mostly electrostatic interaction.
All the experimental vibrational bands have been discussed and assigned to normal modes or to combinations and overtones on the basis of B3LYP and BLYP calculations. With respect to the study of Kalkar et al. , some modes have been reassigned in the light of present theoretical results. It is also shown that a very good quantitative agreement between experimental NMR spectra and the calculated chemical shifts both for 1H and 13C nuclei, can only be reached by modeling the intermolecular interactions. Moreover, theoretical and experimental NMR data suggest that only one of the two protons of NH2 group can be involved in intermolecular hydrogen bonds.
Cuvinte cheie: Pyrazinamide,DFT study, FT-R