Structure of Fluoride Anion Aqueous Solution Derived from X-ray Spectroscopy

We present X-ray emission (XES) and absorption (XAS) spectra of the F^- ion in aqueous solution, along with a theoretical analysis using combined ab initio quantum chemistry spectral calculations and quantum molecular dynamics simulations. The spectra are predicted to have a significantly different dependence on the distance between the anion and the oxygen atoms of water molecules in the first solvation shell. The XES spectrum shows a strong reconfiguration of emission line intensities, while the excitation energy of a resonance in the XAS spectrum is highly sensitive to this distance. Comparisons with the recorded XES and XAS spectra yield predicted F-O distances that agree with 2.7 Å. Two weak features in the XES spectrum are identified as being due to interatomic radiative decay (IRD), thus proving a negative ion counterpart to the recently identified IRD process for the Na⁺ and Mg²⁺ cations in aqueous solution, with the difference that the IRD feature appears on opposite sides of the main ion emission in the two cases. Arguments are provided that the combination of X-ray emission and absorption, together with quantum spectral and molecular dynamics calculations, has much to offer for future studies of the structure and electronic properties of anionic solutions.