NMR Studies on Proton Transfer between Hydration- and Solvent-Water Molecules in Aqueous Solution of AlCl3

Proton resonance at 40.5 Mc for the AlCl 3 solutions was studied at various pH and the temperature down to -29.5°C. The separate signals one for coordinated and the other for bulk water were observed at sufficiently low temperature. The former is separated from the latter to the lower side of magnetic field by 182.6 cps. three kinds of reaction of proton exchanges, are assumed, namely (1) the proton transfer from Al +++ (H 2 O) 6 to H 2 O, (2) the proton transfer from Al +++ (H 2 O) 6 to Al +++ (OH)–(H 2 O) 5 via H 2 O, and (3) the exchange of water molecule between solvation shell of Al +++ and of H + . The corresponding rate constants k 2 and k 3 at -29.5°C in the reactions (2) and (3) were found to be 2.2×10 5 sec -1 mole -1 1H 2 O and 1.8×10 -4 sec -1 mole -1 1H 2 O respectively, and their activation enthalpies Δ H 2 * and Δ H 3 * were 20.9 kcal/mole and 4.9 kcal/mole respectively. The rate due to the reaction (1) was negligible as compared with those of the reaction (2) and (3).