Experimental and density functional theory investigation of some biomedical compounds

Spectroscopic methods (FT-IR, Raman, NMR EPR) and quantum chemical calculations based on density functional theory (DFT) were used for structural characterization of some biomedical compounds with antibiotic, cardiovascular and antimycobacterial activity. The investigated compounds are pyrazinamide (PZA), 5-para-bromo-benziliden-thiazolidin-2-thion-4-one (5pBBTT), atenolol (ATE), metoprolol (MET), pindolol (PIN) and verapamil (VER). The optimized structures of the two possible conformers and the dimer of PZA have been obtained at B3LYP/6-31G(d) level of theory and compared to X-ray data. The experimental vibrational bands (FT-IR and Raman) of PZA were confident assigned on the basis of calculations performed at the same level of theory. Moreover, theoretical and experimental NMR data suggest that only one of the two protons of NH₂ group is involved in intermolecular hydrogen bonds. All the possible conformers and tautomers of 5pBBTT have been investigated by DFT methods and their relative stability is discussed. The very good correlation between the experimental and B3LYP/6-31G(d) theoretical wave-numbers of 5pBBTT allow us to validate the calculated structure of this compound. Both, experimental and theoretical data regarding the NMR spectrum of this molecule suggest the coexistence of both thionic and thiolic tautomers in liquid phase. Powder EPR spectra of ATE, MET, PIN and VER Cu(II) complexes exhibit the absorption signals typical of randomly oriented singlet state (S=1/2) species with axial symmetry. The ground state for paramagnetic electron is d_{x2 - y2} orbital for ATE, MET and VER compounds and d_z2 orbital for PIN compound. Comparing the shape of these EPR spectra with those obtained for other copper complexes with nitrogen and oxygen ligands we have concluded that the local symmetry around metal ions is of square planar type with a CuN₂O₂ chromophore in the xOy plane.