Historically, spectroscopic monitoring of probe solutes has been used to quantify Lewis reactivity. Experimentally studying the intrinsic properties of the Lewis acids and bases themselves without the use of a probe is rarer, and calculations are normally relied upon. Traditionally, Lewis reactivity is explained in terms of electronic structure, especially HOMOs and LUMOs. However, there is currently a lack of understanding of the electronic structure of Lewis acids and bases, mainly because of a dearth of experimental methods for studying the electronic structure of the liquid phase.

In my group, we use a variety of X-ray spectroscopic techniques to link Lewis reactivity from measurements on probes to the intrinsic electronic structure of Lewis acids and bases in the absence of probes, backed up by DFT and ab initio molecular dynamics calculations. Our global aim is to understand and predict Lewis acidity and Lewis basicity of species within the liquid-phase. Most of our X-ray spectroscopy experiments are performed at synchrotrons, including Diamond Light Source in Oxfordshire, and Berlin, Paris, Lund and Barcelona. Synchrotron-based X-ray spectroscopy gives two huge advantages over laboratory-based X-ray spectroscopy: (i) tunability of photon energy, allowing information-rich resonant techniques, (ii) significantly larger flux, allowing low concentrations to be studied, e.g. solutes in solution.

The systems we have focused on are ionic liquids, simple metal complexes, and ions in common molecular liquids. Ionic liquids have the major advantage for X-ray spectroscopy of being non-volatile, meaning they can be studied using standard X-ray spectroscopy apparatus – volatile compounds bring significant experimental challenges. Recently we have expanded our sample set to include organoboron and organozinc compounds. I will present our latest findings, including quantitative links between halometallate complex electronic structure and reactivity, and challenges ahead.