Proton transfer equilibria in Schiff bases with steric repulsion

The proton transfer equilibria in CH2Cl2 solutions of 2-(alpha-(N-methylimino)ethyl)phenol (1), 2-(alpha-(N-methylimino)ethyl)-4,6-dichlorophenol (2) and 2-(alpha-(N-propylimino)ethyl)-4,6-dichlorophenol (3) are studied by UV-vis, H-1 NMR, and FT-IR spectra as a function of temperature. Thermodynamic parameters of the proton transfer reaction have been determined. It has been demonstrated that introducing of the methyl substituent into the C-C(H)=N moiety of the Schiff bases alters the tautomeric equilibrium in favor of the proton transfer forms in comparison with Schiff bases deprived of such substituent and related Mannich bases. Solvatochromy of electronic absorption in Schiff bases is evidence that the proton transfer state is neither zwitterionic nor orthoquinoid in their character but has to be taken as a resonance hybrid of these forms. According to this, a scheme of proton transfer equilibria specific for Schiff bases is proposed, which is in agreement with numerous structural evidences. The B3LYP/6-31G(d,p) frequency calculations performed for both the molecular and proton transfer forms precisely reproduce the experimental spectra measured as a function of temperature. The calculations show that the v(C=N) band assignment, widely discussed in the literature, is ambiguous because of the strong dependence of normal coordinates content on the character and conformation of particular forms of molecules. The calculations definitely show that the protonation of the C=N-R group decreases the force constants of the C=N bonds, in spite of numerous opposite suggestions in the literature.