Kevin LOVELOCK1, Jake SEYMOUR1, Richard FOGARTY2, Ekaterina GOUSSEVA1, Coby CLARKE3, Richard MATTHEWS4, Robert PALGRAVE5, Roger BENNETT1, Patricia HUNT6
1University of Reading, Reading, United Kingdom
2Imperial College London, London, United Kingdom
3University of Nottingham, Nottingham, United Kingdom
4University of East London, London, United Kingdom
5University College London, London, United Kingdom
6Victoria University of Wellington, Wellington, New Zealand
Liquid phase electronic structure is a key factor in chemical reactivity, and thus controls a vast number of liquid phase chemical processes. For ionic liquids (ILs), ionisation energy, Ei, is a crucial descriptor for chemical, photochemical and electrochemical reactivity,1 especially any application that involves exchange of electrons, particularly formal donation of an electron (ionisation) or donation of electron density (partial ionisation). These potential applications include: electrochemical energy storage; gas capture/separation/storage; as solvents for catalysis and metal extraction/separation.2, 3 The identity of the most readily ionised valence state, often called the highest occupied molecular orbital (HOMO), is also a reactivity descriptor. For ILs there is limited experimental data on electronic structure, including Ei and HOMO identity. Ei and the HOMO identity can be used for quantitative validation of calculations of ILs, which is especially useful to test the ability of calculations to capture the solvation effects of ions in liquid phase.4 Currently, electronic structure calculations require expertise in calculations or expensive calculations (or both).
The ionisation energies of 60 ILs are experimentally measured and the most readily ionised valence state of each IL (the highest occupied molecular orbital, HOMO) is identified using a combination of X-ray photoelectron spectroscopy (XPS) and synchrotron resonant XPS.5-7 A structurally diverse range of cations and anions were studied. The cation gave rise to the HOMO for ~10% of the ILs presented here, meaning it is energetically more favourable to remove an electron from the cation than the anion. The cation must be considered as a possible electron donor in such ILs, especially for neutral solutes where electrostatic ion-solute interactions are expected to be less dominant. The influence of the cation on the anion electronic structure (and vice versa) were established; the electrostatic effects are well understood and demonstrated to be predictable. The structurally diverse range of cations and anions studied allow us to provide design rules linking ion structure to valence electronic structure.
Using our experimental data, we show that low-cost density functional theory (DFT) lone ions SMD (Solvation Model based on Density) calculations can be used with a high level of confidence for the prediction of IL valence electronic structure, without the need for input from experimental data (Figure). This key result delivers a significant step towards the computational screening of ILs for many applications. Furthermore, we have the power to carry out calculations on many more ions using lone ions SMD than calculations involving the presence of counterions, offering the potential to make predictions for many ILs with minimal additional computational or user cost.
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