NMR studies on hydrogen bonding and proton transfer in Mannich bases

A review of studies on the ortho Mannich bases containing various substituents in the phenyl ring on the basis of1H,13C and15N nuclear magnetic resonance (NMR) spectra in various solvents over the temperature range 110–298 K is presented. Some new results are also included. The data gathered so far show that there is some critical (inversion) range of ΔpKa (= pKa(NH+) − pKa(OH)) in which the proton transfer equilibrium appears. This inversion range is well reflected in the behaviour of secondary deuterium isotope effect in13C NMR spectra. A strong temperature effect on the strength of hydrogen bonding should be emphasized. The1H chemical shift for trichloroderivative increases from 13.5 at room temperature up to 17 ppm at 130 K when the proton is equally shared between the bridging atoms (1J(1H,15N) = 30–40 Hz). The potential for the proton motion in such bridges is discussed taking into account the behaviour in the ultraviolet and infrared spectra. The role of dimerization in proton transfer equilibria is shown. In addition the rotation of OH groups involved in hydrogen bond formation and nitrogen pyramidal inversion was studied by the1H dynamical NMR spectra.