Charge-transfer complexes of imidazole-2-thiones and N,N′-dialkylthioureas with sulphur dioxide

Abstract

The charge-transfer (CT) complexes of N,N′-dialkylthioureas and of alkyl substituted imidazole-2-thiones with sulphur dioxide in aprotic solvents of different polarity have been studied spectrophotometrically. In chlorinated hydrocarbons of increasing solvent polarity, a blue shift of the CT bands was observed. That indicates that the complexes are more polar in the ground state than in the excited state. For imidazolethiones containing an unsubstituted imine nitrogen, a bathochromic shift was found in n-donor solvents which is explained in terms of the n-donor ability of these solvents. The formation constants and thermodynamic functions, ΔH^o and ΔS^o, have been determined. Deviations in the estimation of formation constants observed in non-polar solvents for all donors except 1,3-dimethylimidazole-2-thione are ascribed to the self-association of donors as a result of hydrogen bonding. Two general numeric procedures for the calculation of self-association constants from spectrophotometric data are proposed and evaluated.