Creatinine and creatininium cation in DMSO-d6 solution. Structure and restricted internal rotation of NH2 group

Investigation of 15N NMR spectra of isotopically enriched creatinine has unequivocally shown that in DMSO-d6 solution it exists as amino tautomer (2-amino-1-methylimidazoline-4-one), which in the presence of acid is protonated at N-3. Free energies of activation of the amino group rotation in creatinine and its cation have been determined to be 56 kJ/mol and 60 kJ/mol, respectively, at 298 K, by performing the simultaneous analysis of the lineshapes of -NH2 proton signals in the whole set of 1H NMR spectra recorded at various temperatures and magnetic fields. These results have been theoretically reproduced by the calculations of molecular structures and energies of creatinine and creatininium cation in their ground states and transition states of the amino group rotation, using DFT (B3LYP) method, involving the influence of the solvent.