PROTON MAGNETIC RESONANCE STUDIES OF PROTON EXCHANGE IN ETHYLENEDIAMINE-WATER AND AMMONIA-WATER SYSTEMS

The exchange of H2O protons with the NH2 protons of ethylenediamine (en) is rapid in the nmr sense over the entire range of composition from pure H2O to pure en. The same is true for NH3 protons and the NH2 protons in en in the NH3-en liquid system. In rather striking contrast, however, the exchange of protons is slow in the H2O-NH3 liquid system at room temperature in solutions containing 0 to ∼50 mole % H2O. Addition of small amounts of NH4+ markedly accelerates proton exchange in this system. In both the H2O-en and H2O-NH3 systems, the averaged H2O-amine proton resonance displays significant paramagnetic deviations from the straight line which assumes weighted averaging of the proton shifts for each pure component. The deviation is a maximum in the region of composition corresponding to one H2O per amine group. These results suggest an amine-hydrate liquid structure in which hydrogen bonding is stronger than a simple average between the hydrogen bonding in pure H2O and in pure amine. A study of the kinetics of H2O-NH3 proton exchange at low H2O concentrations in the presence of added NH4+ shows that the proton exchange between H2O and NH3 is best fit by a kinetic expression of the form k[NH4+][H2O] in which k = 3.48 × 106 (moles/l.)-1 sec-1, with a standard deviation of ∼1%. Although more than one interpretation is possible, these results are consistent with a mechanism involving proton exchange between an NH4+ ion and a water-ammonia solvate.