Charge Transfer Complexes Formed by Heterocyclic Thioamides and Tetracyanoethylene: Experimental and Theoretical Study

Tetracyanoethylene (TCNE) as one of the most versatile organic compounds is involved in various chemical reactions with electron transfer. Charge transfer complexes (CTCs) of a few antioxidants, nitrogen containing thioamides [pyrrolidine-2-thione (I), 1,3-H-imidazolidine-2-thione (II), 1,3-H-Imidazoline-2-thione (III), pyridine-2-thione (IV), 5-trifluoromethylpyridine-2-thione (V), 4-trifluoromethylpyrimidine-2-thione (VI), quinoline-2-thione (VII), 3,4,5,6-tetrahydropyrimidine-2-thione (VIII)] as π-donors and TCNE as π-acceptor were studied. The DFT PCM/UB3LYP/6-31++G­(d,p) and SA-CASSCF quantum chemical calculations were used to study the structures and relative stabilities of these complexes in the ground and lowest excited electronic states. The formation of a weak molecular associates in the chloroform and acetonitrile solutions was confirmed by UV/vis and IR absorption spectroscopy. The stability constants and molar extinction coefficients were estimated by UV/vis spectroscopy. The highest stability in acetonitrile is found for associates formed by quinoline-2-thione and pyridine-2-thione with TCNE, the lowest one is found for CTC formed by imidazolidine-2-thione. Molecular associate formed by pyridine-2-thione and TCNE has the greatest stability in the chloroform solution. 5-Trifluoromethylpyridine-2-thione and 4-trifluoromethylpyrimidine-2-thione do not form CTC in CH3CN due to the presence of an electron acceptor group in the molecules. The molar extinction values of CTC vary within the range of 0.4 × 103 to 1.0 × 104 M–1 cm–1. An analytical strategy of thioamides identification based on wavelength and intensity of CTCs absorption band has been suggested.