Theoretical Study On The Water Autodissociation Process In Small Clusters

 

Jiří Mrázek and Jaroslav V. Burda

 

Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic

 

Despite large progress in computational facilities, obtaining of reliable hydration free energies still presents a difficult task. In this work, a model of hydration based on explicit treatment of water molecules was investigated. Product state of the autodissociation process – the ion-pair formation was modeled in small water clusters (up to 21 monomers) using correlated ab-initio methods. The influence of cluster size and geometry on the resulting pKa (pH) value was examined. The pH values decrease with increasing cluster size going from pH~25 in the dimer case to pH~5 for hexamer. For the correct pH determination, the Boltzmann weighting of all the assumed clusters has to be considered. Topological arrangement of H-bonds rather then cluster size determines the pH values of larger clusters. The pH values range from 6 to 10.

Calculations based on the largest 21-water structure indicate that the ion-pair formation inside the cluster requires significantly higher energy compared to the ion formation on the cluster surface.

It was also found a correlation between the distance between the monomers involved in proton transfer and the free energy of the autodissociation process. This enables us to predict (or estimate) the pH values without very demanding calcualtions.