Water-ammonia and water-acetonitrile proton transfer free energy

Abstract

Proton transfer at the interface of solvents are of great importance in chemistry and biology. Despite their importance, there are very rare computational models that do describe the proton transfer processes. In this work, we propose a simple and affordable yet efficient model to compute the proton transfer free energy and enthalpy. We applied the proposed model to the calculation of the water-ammonia as well as water-acetonitrile proton transfer free energy and enthalpy. To compute the water-ammonia proton transfer free energy and enthalpy within this model, we need the structures of neutral and protonated water clusters as well as the structures of neutral and protonated ammonia clusters optimized in the solvent phases. Thus we started by exploring the potential energy surfaces (PESs) to locate the most stable structures of the clusters involved in the model. We initially generated the geometries using the ABCluster code followed by full optimization at the MN15/6–31++G(d,p) level of theory combined with the polarized continuum model. As a result, we have reported the global minima energy structures of the investigated clusters in the solvent phase. At room temperature, the water-ammonia (respectively water-acetonitrile) proton transfer free energy and enthalpy are evaluated to be −109.9 and −104.2 kJ mol−1 (respectively 60.2 and 58.0 kJ mol−1), respectively. The calculated water-ammonia as well as water-acetonitrile proton transfer free energy and enthalpy are found to be in good agreement with experimental values when available.