Stability of N-(ortho-hydroxynaphthylmethylene)methylamines and their tautomers

Thermodynamic parameters of the proton transfer equilibrium for three isomeric N-(ortho-hydroxynaphthylmethylene)methylamines have been determined by use of the temperature NMR data and deuterium isotope effects on 13C chemical shifts for their solutions in CDCl3, CD3CN and DMSO-d6. The DFT geometry optimization as well as calculation of the energies, dipole moments and HOMA (aromaticity) parameters for the OH and NH tautomers in vacuum and in solution has been performed. Interpretation of origin of the observed differences in the equilibrium constants for the three isomers studied is based on the differences in competition between π-electron delocalization in the quasi-ring involving the intramolecular hydrogen bond, differences in the polarity and on the peri interactions.