Tautomeric behaviors of 5-arylazobarbituric acids in different concentrations

The NMR spectra of azo dyes, 5-arylazobarbituric (5a-g), 5-arylazo-1,3-dimethylbarbituric(6a-g) and 5-arylazothiobarbituric acids (7a-g) were studied in DMSO-d6 in differentconcentrations. An intramolecular hydrogen bond was observed and indicating that thehydrazone forms is mostly predominant. The peak of the hydrazone proton was severelybroadened and its chemical shift appeared at down field due to intramolecular hydrogen bond.Existence of nitro group at ortho-position on phenyl ring caused the chemical shift value of theproton of hydrazone form in 5a-7a to be more deshielded than other hydrazone protons in 5bg−7b-g due to bifurcated intramolecular hydrogen bond and anisotropic ring-current effect. Dyes6 shows two tautomers in NMR time scale at low concentration in DMSO-d6.