Lone pair versus bonding pair electrons: the mechanism of electronic polarization of water in the presence of positive ions.

It is commonly accepted that the water molecules in the first solvation shell of a positive ion are strongly polarized because of an elongation of the oxygen lone pair orbitals along the ion-oxygen direction and this is commonly considered the dominant effect. Recent experimental and theoretical works have instead suggested that this is not the dominant aspect and that the problem is by far more complicated. Consistent with the picture given above, here we show that, in particular, an equally important role into the polarization process is played by the bonding pair electrons located along the internal oxygen-hydrogen bond. We also provide some arguments which suggest that the main reason of such a behavior is due to the distortion of the molecular orbitals caused by the interaction between non-hydrogen-bonded water molecules in the first solvation shell of the ion.