Structures, energetics, and spectra of aqua-sodium(I): Thermodynamic effects and nonadditive interactions

Abstract

Using extensive ab initio calculations including electron correlation, we have studied structures, thermodynamic quantities, and spectra of hydrated sodium ions [Na(H2O)+n (n=1–6)]. Various configurations were investigated to find the stable structures of the clusters. The vibrational frequency shifts depending on the number of water molecules were investigated along with the frequency characteristics depending on the presence/absence of outer-shell water molecules. The thermodynamic quantities of the stable structures were compared with experimental data available. Entropy-driven structures for n=5 and particularly for n=6 are noted in the calculations, which can explain the peculiar experimental thermal energies. On the other hand, the enthalpy effect to maximize the number of hydrogen bonds of the clusters with the surrounding water molecules seems to be the dominant factor to determine the primary hydration number of Na+ in aqueous solution. The nonadditive interactions in the clusters are found to be large. To have reliable binding energies, up to four-body interactions need to be included. Nevertheless, as a good approximation to calculate the total electron correlation energy, only water–water two-body interactions can be taken into account because other many-body interactions are partially offset.