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Study Reveals Orbital Mixing Between Water and Dissolved Ions

The essential role of water as a solvent in chemistry and biology is closely connected to the chemical interactions between dissolved ions and the water molecules immediately surrounding them (the "first hydration shell"). However, selectively measuring the electronic structure of such water molecules out of all the molecules in the solution has been a formidable challenge. With recent developments in soft x-ray absorption spectroscopy (XAS) applied to liquids under ambient conditions, such measurements are now possible. By combining these measurements, taken at ALS Beamline 8.0.1, with density functional theory (DFT) calculations, researchers from Sweden and the U.S. have demonstrated that the molecular orbitals of the water molecules in the first hydration shell will mix with the d orbitals of a dissolved transition-metal ion, resulting in characteristic pre-edge features in the oxygen 1s XAS spectrum. The technique is sensitive enough to easily detect an additional chlorine ion in the hydration shell.
hydration shells
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.

The experimental O 1s XAS spectrum of water with dissolved chromium chloride [CrCl3(aq)] has a pre-edge region (532–536 eV) containing features not seen in the spectrum of water with dissolved aluminum chloride [AlCl3(aq)]. This difference arises because the chromium's empty d states (characteristic of transition metals) cause it to bond differently with the surrounding water molecules than the (non-transition-metal) aluminum. A subtraction procedure enhances the differences and reveals three peaks.

A Water Solution

experimental difference spectrum and calculated spectra

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.

extra pre-edge features

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).

ALSNews Vol. 234, November 12, 2003

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