Transition-metal ion (center) surrounded by 18 water molecules: 6 in the first hydration shell and 12 in the second. In XAS, an incident beam of photons can be used to selectively probe the electronic structure of water molecules in the first hydration shell.

A Water Solution

Water is a deceptively simple molecule. Three atoms, two covalent bonds, and a slight dipole moment combine to form a substance with many unusual properties that make it essential to life. It is a coolant, a heat reservoir, an acid, a base, a nearly universal solvent, and a weak electrical conductor; it plays a wide range of roles from regulating the Earth's climate to affecting the way proteins fold. Many of water's unusual properties arise from its propensity to form hydrogen bonds with nearby molecules. Hydrogen bonds are relatively "loose," based on attractions between opposite electrical charges. To better understand what happens when such bonds are formed between water molecules and dissolved ions, Näslund et al. studied the electronic structure of oxygen using an x-ray absorption spectroscopy (XAS) technique that allows measurement of aqueous solutions at atmospheric pressure and room temperature with great sensitivity to the local environment of the dissolved ions. With this technique, combined with a powerful computational theory, the researchers gained insight into the local electronic structure of a hydrogen-bonded liquid and were able to identify the "fingerprint" of the molecules in direct contact with the solvated ion.

Features in the difference spectrum between CrCl₃(aq) and AlCl₃(aq) (red) and in the calculated spectra of [Cr(H₂O)₆]³⁺ (green) and [CrCl(H₂O)₅]²⁺ (blue) serve as a spectroscopic proof of d-orbital interaction between the Cr³⁺ ion and the water molecules in the first hydration shell.

DFT calculations for the chromium-water cluster [Cr(H₂O)₆]³⁺ assign the two features at 533.8 and 535.8 eV to molecular orbitals having strong metal d character. In the computed spectrum of a similar complex in which one chloride replaces a water molecule in the first hydration shell [CrCl(H₂O)₅]²⁺, the spectrum is shifted upward by about 1 eV, generating a feature at 534.5 eV. The conditions of the experiment were such that an equal mixture of [CrCl(H₂O)₅]²⁺ and [Cr(H₂O)₆]³⁺ must be expected, and this expectation is beautifully confirmed in the resulting soft x-ray spectrum.

To show that the mixing between the molecular orbitals of water and the Cr³⁺d orbitals represents a specific example of a more general phenomenon, the researchers also studied solutions of another transition metal, iron, which has a much more complicated solution chemistry. The bonding of the Fe³⁺ ion is strong enough to cause deprotonation of the surrounding water molecules, resulting in hydroxide ions (OH^–) in the first hydration shell. Alternatively, the addition of hydrochloric acid to the solution lowers the pH, inhibits deprotonation, and promotes the presence of chlorine ions in the first hydration shell. Spectra of a sequence of Fe³⁺ complexes allow a definite assignment of the distinct and complicated d-orbital features at 530.0, 531.6, and 532.8 eV in the FeCl₃(aq) spectrum. Computed x-ray absorption spectra indicate that the first peaks in the FeCl₃(aq) spectrum, at 530.0 and 531.6 eV respectively, are due to the interaction between OH^– molecular orbitals and d orbitals in the metal. This is confirmed by the absence of these two peaks in the low-pH FeCl₃ spectrum (where OH^– is replaced by Cl^–). The broad peak at 532.8 eV is then assigned to the d-interaction of the water molecules in the Fe³⁺ ion hydration shell. Overall, it was possible to assign all the peaks in the spectrum based on computed spectra for various possible compositions of the first hydration sphere.

Experimental O 1s x-ray absorption spectra of various aqueous solutions. Extra pre-edge features (shaded areas) only appear if the dissolved ion is a transition metal. Differences between the spectra of the various Fe³⁺ complexes are due to the interaction between OH^– orbitals and d orbitals in the metal. Also shown is the O 1s x-ray absorption spectrum of pure water, with features attributable to three configurations (symmetric and asymmetric) of water molecules.

Although the interaction between the water and the transition-metal d orbitals was anticipated in the literature, until now, there was no direct exerimental evidence for it. XAS, combined with DFT calculations, is the only technique sensitive and selective enough to directly probe local orbital changes resulting from such a weak interaction. Soft x-ray measurements on ionic solutions have thus been demonstrated to provide unique information on the electronic structure, bonding, and composition in the first hydration shell of dissolved ions.

Research conducted by L.-Å. Näslund (Stockholm University and Uppsala University); M. Cavalleri, H. Ogasawara, L.G.M. Pettersson, and M. Sandström (Stockholm University); A. Nilsson (Stockholm University and Stanford Synchrotron Radiation Laboratory); P. Wernet (Stanford Synchrotron Radiation Laboratory); and D.C. Edwards and S. Myneni (Princeton University).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); the Swedish Royal Academy of Science; the Göran Gustafsson Foundation; the National Science Foundation; the Swedish Natural Science Research Council; the Foundation for Strategic Research; and the Swedish Center for Parallel Computing. Operation of the ALS is supported by BES.

Publication about this research: L.-Å. Näslund, M. Cavalleri, H. Ogasawara, A. Nilsson, L.G.M. Pettersson, P. Wernet, D.C. Edwards, M. Sandström, and S. Myneni, "Direct Evidence of Orbital Mixing between Solvated Transition-Metal Ions: An Oxygen 1s XAS and DFT Study of Aqueous Systems," J. Phys. Chem. A, 107, 6869 (2003).