Proton exchange in ammonia, water and formic acid dimers: quantum-chemical calculation

Proton exchange in hydrogen-bounded complexes occupies an important place among dynamic processes taking place in molecular systems with hydrogen bond. However, despite numerous experimental and theoretical studies in this field, a single point of view on the mechanism of proton exchange has not yet been accepted by scientists. Ammonia, water and formic acid are small in size protolytes with widely differing acid-base properties. This makes them suitable and comfortable for theoretical modeling of proton exchange reaction. Quantum-chemical simulation of the proton exchange reaction in model dimers of ammonia, water and formic acid was carried out by AM1 and ab initio 6-31G, 6-31 G++ methods of Gaussian-2009 program. The search of transition state structure was performed by using of QST2 procedure, the descent along the reaction coordinate was held by using of IRC procedure. The symmetrical structure of transition state in the case of formic acid dimer and the asymmetric structure of transition complex in the case of ammonia dimer were obtained for studied proton exchange reaction. A synchronous mechanism of proton exchange reaction is shown in the case of the formic acid dimer and a sequential mechanism is shown in the case of ammonia dimer. The dynamic shortening of the hydrogen bridge length was noted during proton exchange reaction in all model systems. It was suggested that the mechanism of proton exchange reaction is determined by the nature of the resulting transition state (symmetrical or asymmetrical). At the same time, the transition state structure is determined by the acid-base properties of reaction partners.