Tautomeric behaviour and isotopic multiplets in the 13C NMR spectra of partially deuterated 5-arylazo-pyrimidine (1H,3H,5H)-2,4,6-triones and 5-arylazo-2-thioxo-pyrimidine (1H,3H,5H)-4,6-diones-evidence for elucidation of tautomeric forms.

NMR spectra of the synthesized azo dyes, 5-arylazo-pyrimidine (1H,3H,5H)-2,4,6-triones (5a-g), 1,3-dimethyl-5-arylazo-pyrimidine (1H,3H,5H)-2,4,6-triones (6a-g), and 5-arylazo-2-thioxo-pyrimidine (1H,3H,5H)-4,6-diones (7a-g) were studied in (CD(3))(2)SO (three drops of CD(3)OD were added into solutions of the dyes in two different concentrations). All dyes showed intramolecular hydrogen bonding. Dyes 5a-7a showed bifurcated intramolecular hydrogen bonds. Tautomeric behaviours of some of N-methylated azo dyes (6a-g) were studied in two different concentrations. The solvent-substrate proton exchange of dyes 5a-d, 6a and 7a-e was examined in presence of three drops of CD(3)OD. The dyes which were soluble in (CD(3))(2)SO containing CD(3)OD showed isotopic splitting (beta-isotope effect) in the (13)C NMR spectra.