Investigation of solvated calcium dication structure in pure water, methanol, and ethanol solutions by means of K and L2,3-edges X-ray absorption spectroscopy

Abstract

The interaction of an ion with surrounding solvent molecules is revealed by X-ray absorption spectroscopy in transmission mode for the case of calcium dication dissolved in pure water, methanol and ethanol at the Ca K and L2,3-edges. The near-threshold K-shell excitation leads to two distinct solvent-dependent features, the Ca 1s to 3d and 4p electronic transitions, clearly indicating the ligand field decreases as water is changed to methanol and methanol to ethanol. On the other hand, the Ca L2,3-edge excitation leads to small changes in the absorption spectra and the peak assignment – mostly in terms of Ca 2p to eg and t2g transitions for each of the two spin orbit components – is strongly depending on the geometry of the first solvation shell around the dication and on the magnitude of the ligand-field multiplet effects. Our interpretation is supported by ab-initio quantum chemical calculations of the solvent dependence of chemical shifts for the Ca 3d and 4p unoccupied levels of the 1s and 2p excited states, performed on a small cluster including the nearest neighbor solvent molecules surrounding the Ca dication assuming a cubic (8-fold) solvent coordination.