COLLECTIVE PROTON MOTION AND PROTON POLARIZABILITY OF THE HYDROGEN-BONDED SYSTEM IN DISUBSTITUTED PROTONATED MANNICH-BASES

Nine 2-mono- and 2,6-disubstituted Mannich bases with different 3- and 4-substituents as well as the monoperchlorates of the protonated disubstituted compounds were studied by Fourier-transform infrared spectroscopy. In the case of the mono-and disubstituted Mannich bases the intensity of an infrared (IR) continuum increases with increasing acidity of the phenolic group indicating increasing proton polarizability of the intramolecular OH⋯N ⇌ O-⋯H+N hydrogen bonds. In the case of the monoperchlorates of the disubstituted Mannich bases with increasing acidity of the phenolic group the intensity of the IR continuum first increases indicating collective proton fluctuation and proton polarizability of the N+H⋯OH⋯N ⇌ N⋯HO⋯H+N system. If the acidity increases further, a third proton limiting structure N+H⋯O-⋯H+N yields weight. The IR continuum indicates now collective proton motion and proton polarizability in the N+H⋯OH⋯N ⇌ N+H⋯O-⋯H+N ⇌ N⋯HO⋯H+N system. With further increasing acidity of the phenolic group, the intensity of the continuum decreases. This result indicates that the proton polarizability decreases again. Finally, the protons are localized at the N atoms; i.e., only the proton limiting structure N+H⋯O-⋯H+N is realized. © 1990 American Chemical Society.