Unravelling the complex speciation of halozincate ionic liquids using X-ray spectroscopies and calculations

Using a combination of liquid-phase experimental X-ray spectroscopy experiments and small-scale calculations we have gained new insights into the speciation of halozincate anions in ionic liquids (ILs). Both core and valence X-ray photoelectron spectroscopy (XPS) were performed directly on the liquid-phase ILs, supplemented by Zn 1s X-ray absorption near edge structure (XANES) spectroscopy. Density functional theory (DFT) calculations were carried out on both 1- and 2- halozincate anions, in both a generalised solvation model SMD (Solvation Model based on Density) and the gas phase, to give XP spectra and total energy differences; time-dependent DFT was used to calculate XA spectra. Speciation judgements were made using a combination of the shape and width of experimental spectra, and visual matches to calculated spectra. For 2- halozincate anions, excellent matches were found between experimental and calculated XP spectra, clearly showing that only 2- halozincate anions were present at all zinc halide mole fraction, x, studied. At specific x (0.33, 0.50, 0.60) only one halozincate anion was present; equilibria of different halozincate anions at those x were not observed. All findings show that chlorozincate anion and bromozincate anion speciation matched at the same x. Based on the results, predictions are made of the halozincate anion speciation for all x up to 0.67. Caution is advised when using differences in calculated total energies obtained from DFT to judge halozincate anion speciation, even when the SMD was employed, as predictions based on total energy differences did not always match the findings from experimental and calculated spectra. Our findings clearly establish that the combination of high-quality experimental data from multiple spectroscopies and a wide range of calculated structures are essential to have high confidence in halozincate anion speciation identification.