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

The electronic structure of model aqueous solutions of Na+ and Ag+ is inves tigated using ab initio molecular-dynamics methods. We compute a number of electronic response coefficients in solution, such as global hardness and n uclear Fukui functions. The nuclear Fukui functions are found to be particu larly 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 resul t 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 cons truction of effective molecular orbitals and energies. This analysis reveal s that the effective highest occupied molecular orbital (HOMO) of the hard cation, Na+, remains buried in the valence bands of the solvent, whereas th e HOMO of Ag+ is found to mix with the lone pair electrons of its four liga nd H2O molecules to form the (global) HOMO of the solution. This observatio n, highlighting the importance of the electronic structure of the solvent, is used to rationalize the results for the electronic response. (C) 2001 Am erican Institute of Physics.