Efficient prediction of the local electronic structure of ionic liquids from low-cost calculations

Understanding and predicting ionic liquid (IL) electronic structure is crucial for their development, as local, atomic-scale electrostatic interactions control both the ion-ion and ion-dipole interactions that underpin all applications of ILs. Core-level binding energies, E_B(core), from X-ray photoelectron spectroscopy (XPS) experiments capture the electrostatic potentials at nuclei, thus offering significant insight into IL local electronic structure. However, our ability to measure XPS for the many thousands of possible ILs is limited. Here we use an extensive experimental XPS dataset comprised of 44 ILs to comprehensively validate the ability of a very low-cost and technically accessible calculation method, lone-ion-SMD (solvation model based on density) density functional theory (DFT), to produce high quality E_B(core) for 14 cations and 30 anions. Our method removes the need for expensive and technically challenging calculation methods to obtain E_B(core), thus giving the possibility to efficiently predict local electronic structure and understand electrostatic interactions at the atomic scale. We demonstrate the ability of the lone-ion SMD method to predict the speciation of halometallate anions in ILs.