Direct evidence of orbital mixing between water and solvated transition-metal ions: An oxygen 1s XAS and DFT study of aqueous systems

We have studied the chemical bonding of water in the first hydration sphere to transition-metal ions in aqueous solutions by using X-ray absorption spectroscopy (XAS) combined with density functional theory calculations (DFT). The experimental technique is for the first time applied to the study of the oxygen K-edge absorption of liquid water in the presence of dissolved ions. We successfully resolved the electronic structure of water in the first hydration sphere. Features in the oxygen 1s XAS pre-edge region in Cr³⁺ and Fe³⁺ solutions are interpreted as mixing between the molecular orbitals of water and the open d shell of the transition-metal ion. These features are absent for Al³⁺ solutions. Effects on the electronic structure from anions in the first hydration sphere are also described.