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.

Can we understand the different coordinations and structures of closed‐shell metal cation‐water clusters?

“…And finally, one real limitation is that without previous knowledge of the CN, the system to consider cannot be accurately defined. For example, according to a lot of theoretical studies, 2 the most stable structure of Na(H 2 O) 6 ϩ clusters is a (4 ϩ 2) one (i.e., four molecules directly coordinated to the cation), whereas in dilute solution the CN of Na ϩ is known to be 6. Hence, we have to consider an isomer that has a higher energy and apply a PCM model if we hope to accurately reproduce the bulk properties by ab initio calculations.…”

Three-body Effects in Calcium(II)-ammonia Solutions: Molecular Dynamics Simulations

“…Furthermore, Lombardi and Jansen [3] also extended the approach to four-body interactions and found that these effects were negligible for most geometries of interest. These observations have been illustrated for a monatomic gas [4] and numerous cases of ions in aqueous solution [5][6][7][8]. In addition, recent studies of Na(I), Mg(II), or Zn(II) solvation in liquid ammonia by Monte Carlo simulations showed that the three-body corrections reduce their first shell coordination number from 9 to 8, 10 to 6, or 9 to 6, respectively [9][10].…”

On the accuracy of density functional theory for ion–molecule clusters. A case study of PLn+ clusters of the first and second row hydrides

“…Inclusion of electron correlation (3)(4)(5)(6), at least at the MP2 level, is essential to describe the bonds in such systems, and normally the use of a large basis set is necessary to yield accurate binding energies. This makes the computational cost too expensive.…”