The 15N NMR study of the tautomeric equilibria of three creatinine derivatives in solution and in the solid state

Three (I, II and III) creatinine derivatives (Scheme 1) have been investigated by N-15 NMR method in DMF solution at low temperature and in the solid state to determine which tautomeric form (A, B or C) is the most abundant one in the specified condition. It was found that derivatives I and 11 in solution exist as a mixture of three tautomers (the 1-NH (A) form is predominant one) whereas the COOH derivative exists as a pure 3-NH (13) tautomer. In the solid state only the I-NI I forms with different strength hydrogen bonds were observed for compounds I and II. For compound III in the solid state the complete proton transfer from carboxylic group to imidazole ring leading to the zwitterionic structure was determined.