Electronic properties of hard and soft ions in solution: Aqueous Na+ and Ag+ compared

Abstract

The electronic structure of model aqueous solutions of Na+ and Ag+ is investigated using ab initio molecular-dynamics methods. We compute a number of electronic response coefficients in solution, such as global hardness and nuclear Fukui functions. The nuclear Fukui functions are found to be particularly sensitive to the chemical nature of the component species giving for Ag+ a susceptibility 3.5 times the value for a H2O molecule while the result for Na+ is more than a factor of 4 smaller compared to a solvent molecule. The electronic structure of the solution is further characterized by construction of effective molecular orbitals and energies. This analysis reveals that the effective highest occupied molecular orbital (HOMO) of the hard cation, Na+, remains buried in the valence bands of the solvent, whereas the HOMO of Ag+ is found to mix with the lone pair electrons of its four ligand H2O molecules to form the (global) HOMO of the solution. This observation, highlighting the importance of the electronic structure of the solvent, is used to rationalize the results for the electronic response.