An<i>ab initio</i>LCAO-MO-SCF study of reaction paths for proton transfer in ammonium aqueous solution

Abstract

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. Contribution from the laboratoire de Chimie Physique Organique, (E.R.A. No. 222), Université de Nancy I, C.O. 140, 54037 Nancy Cedex, France, and the laboratoire de Chimie Quantique, E.R. No. 139 du CNRS, Université Louis Pasteur, B.P. 296/R8, 67000 Strasbourg, France. Contribution from the laboratoire de Chimie Physique Organique, (E.R.A. No. 222), Université de Nancy I, C.O. 140, 54037 Nancy Cedex, France, and the laboratoire de Chimie Quantique, E.R. No. 139 du CNRS, Université Louis Pasteur, B.P. 296/R8, 67000 Strasbourg, France. Notes Contribution from the laboratoire de Chimie Physique Organique, (E.R.A. No. 222), Université de Nancy I, C.O. 140, 54037 Nancy Cedex, France, and the laboratoire de Chimie Quantique, E.R. No. 139 du CNRS, Université Louis Pasteur, B.P. 296/R8, 67000 Strasbourg, France.