Abstract
Ionic liquid (IL) valence electronic structure provides key descriptors for understanding and predicting IL properties. The ionisation energies of 60 ILs are 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. A structurally diverse range of cations and anions were studied. The cation gave rise to the HOMO for nine of the 60 ILs presented here, meaning it is energetically more favourable to remove an electron from the cation than the anion. 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 consistently predictable. We used this knowledge to make predictions of both ionisation energy and HOMO identity for a further 516 ILs, providing a very valuable dataset for benchmarking electronic structure calculations and enabling the development of models linking experimental valence electronic structure descriptors to other IL properties, e.g. electrochemical stability. Furthermore, we provide design rules for the prediction of the electronic structure of ILs.
Highlights
Ionisation energy, Ei, is a key descriptor for chemical, photochemical and electrochemical reactivity,[1,2,3,4,5] especially any application that involves exchange of electrons, formal donation of an electron or donation of electron density
The identity of the most readily ionised valence state, often called the highest occupied molecular orbital (HOMO),[12] is a reactivity descriptor, for Ionic liquid (IL) given the HOMO could come from the anion or from the cation
We have studied a total of 60 ILs using laboratorybased X-ray photoelectron spectroscopy (XPS); 37 ILs from ref. 48 and 23 ILs newly published here. 18 of these ILs were studied using synchrotron resonant XPS; 7 ILs from ref. 48 and 11 ILs newly published here
Summary
Ionisation energy, Ei, is a key descriptor for chemical, photochemical and electrochemical reactivity,[1,2,3,4,5] especially any application that involves exchange of electrons, formal donation of an electron (ionisation) or donation of electron density (partial ionisation). For ionic liquids (ILs), these potential applications include: electrochemical energy storage; gas capture/separation/storage; as solvents for catalysis and metal extraction/separation.[6,7,8,9,10,11] The identity of the most readily ionised valence state, often called the highest occupied molecular orbital (HOMO),[12] is a reactivity descriptor, for ILs given the HOMO could come from the anion or from the cation. For ILs there is limited experimental data on electronic structure, including Ei and HOMO identity
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