Self-association, tautomerism and E–Z isomerization of isatin–phenylsemicarbazones – spectral study and theoretical calculations

摘要

The self-association and tautomerism of (E)-isatin-3-4-phenyl(semicarbazone) Ia and (E)-N-methylisatin-3-4-phenyl (semicarbazone) IIa were investigated in solvents of various polarity. In weakly interacting non-polar solvents, such as CHCl_3 and benzene, phenylsemicarbazone concentrations above 1×10~(-5) mol dm~(-3) result in the formation of dimers or higher aggregates of E-isomers Ia and IIa. This aggregate formation prevents room temperature E–Z isomerization of la and IIa to more stable Z-isomers. In contrast to the situation in non-polar solvents, E–Z isomerization from the monomeric form of phenylsemicarbazone Ia and IIa E-isomers occurs in highly interactive polar solvents including MeOH and DMF only at temperatures above 70 °C. Moreover, decrease in phenylsemicarbazone concentration below 1×10~(-4) mol dm~(-3) in these highly solute–solvent interacting systems leads to aggregate dissociation, and a new hydrazonol tautomeric form with a high degree of conjugation predominates in these solutions. Theoretical calculations confirm obtained experimental results.