Deprotonation of amides and polyfunctional imides probed by heteronuclear NMR and quantum chemical calculations

The site of deprotonation of several types of amide acids (carboxylic amide s and imides, sulfonamides, cyanamide, N-hydroxyurea) has been investigated by quantum chemical calculations and heteronuclear NMR measurements. Relat ive energies of tautomeric ions deriving from protonation at the various si tes were determined both in the gas phase and in water (by the IPCM continu um solvation method). NMR properties (nuclear shielding and electric field gradient) of the involved heteronuclei were calculated and compared with ex perimental N-14, O-17 and S-33 chemical shifts and relaxation rates. It is shown that the combination of theoretical and experimental tools allows a r eliable prediction of spectral parameters and ultimately of the deprotonati on site of polyfunctional acids.