Anab initioLCAO-MO-SCF study of reaction paths for proton transfer in ammonium aqueous solution

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Abstract
The possible mechanisms for proton transfer in ammonium aqueous solutions are discussed through ab initio LCAO-MO-SCF calculations for the following hydrogen-bonded complexes : [NH4 + … NH3] ; [NH4 + … OH2] ; [NH4 + … OH2 … OH2] ; [NH4 + … OH2 … NH3] and [H2O … NH4 + … OH2 … OH2]. The energy curve along the reaction coordinate is drawn for the first three systems. A double well potential curve is obtained for the two symmetrical systems with a very low barrier to proton transfer : 2·9 kcal/mole for the system [NH4 + … NH3] and 4·3 kcal/mole for the system [NH4 + … H2O … NH3]. For both systems the exchange mechanism involves three successive steps : association, transfer and dissociation. Solvation may affect the energetics of the first and third steps. For the unsymmetrical system NH4 + + H2O, the energy would increase continuously during the steps of proton transfer and dissociation. Hence the process of proton transfer between an ammonium ion and a water molecule may take place in solution only if assisted either by solvation or by a concerted push-pull mechanism involving a third molecule [NH4 + … OH2 … NH3]. Theoretical results for the systems [NH4 + … OH2 … OH2] and [NH3 … H3O+ … H2O] show, indeed, that solvation should make the proton transfer easier. In any case the proton transfer is found to occur through a contraction of the associated species formed in the first step.

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