Estimation of σ- and π-donor properties of heterocyclic thioamides by spectroscopic and magnetic resonance methods

The charge-transfer complexes (CTC) of few thioamide: 1-methylimidazoline-2-thione (MMI), 3-methyl-1-ethoxycarbonilimidazoline-2-thione (Carb), 5-methylbenzimidazoline-2-thione (BIZ), benzothiazoline-2-thione (BTZ), benzoxazoline-2-thione (BOZ) as σ-donors and diiodine as σ-acceptor were studied by spectroscopic methods (UV/Vis, 1H NMR). CTC formation constants of thioamides with diiodine were determined using the function of the average-iodine number.The charge-transfer complexes of thioamides as π-donors with tetracyanoethylene (TCNE) as π-electron acceptor, were studied by UV-spectroscopy in dichloromethane and chloroform solutions. The mechanism of interaction MMI and Carb with TCNE have been studied by EPR spectroscopy. Spectral characteristics and formation constants are discussed in the terms of electron donor affinity of thioamides and the nature of the organic solvent used. The ionization potentials of donors were estimated from the CT transition energies of their complexes.The photolytic equilibrium constants of five thioamides are determined using pH-metric titrations.

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► The acid–base properties of drug molecules are a key parameter for drug development, we investigated the protolytic properties of the thioamides.
► CTC formation constants of thioamides with diiodine were determined using the function of the average-iodine number.
► Correlation between base properties and stability of complexes with diiodine is observed for benzoannulated derivatives thioamides.
► The EPR data show sharp reduction of concentration of radicals in the solution and powder samples of Carb complex with TCNE in comparison with a complex of MMI with TCNE.