Deuterium isotope effect on C-13 NMR spectra of ortho Mannich bases

The deuterium isotope effect upon C-13 NMR spectra was studied for ortho Mannich bases of varying Delta pK(a), value [Delta pK(a) = pK(a)(NH+) - pK(a)(OH)]. The Delta values [Delta = delta(13)C(OH) - delta(13)C(OD)] for different positions in the phenyl ring are consistent with calculations of charge distribution by means of MNDO - PM3 method and assuming that deuteration leads to a weakening of the hydrogen bonding. In the critical range of Delta pK(a), where the proton transfer appears, the inversion of the sign of the isotope effect for the carbon atom in position 1 takes place. Strong dependence of the Delta value on temperature was revealed for 2-(N,N-diethylaminomethyl)-3,4,5,6-tetrachlorophenol showing the proton transfer equilibrium.