Valence electronic structure is crucial for understanding and predicting reactivity. Valence non-resonant X-ray photoelectron spectroscopy (NRXPS) provides a direct method for probing the overall valence electronic structure. However, it is often difficult to separate the varying contributions to NRXPS; for example, contributions of solutes in solvents or functional groups in complex molecules. In this work we show that valence resonant X-ray photoelectron spectroscopy (RXPS) is a vital tool for obtaining atomic contributions to valence states. We combine RXPS with NRXPS and density functional theory calculations to demonstrate the validity of using RXPS to identify atomic contributions for a range of solutes (both neutral and ionic) and solvents (both molecular solvents and ionic liquids). Furthermore, the one-electron picture of RXPS holds for all of the closed shell molecules/ions studied, although the situation for an open-shell metal complex is more complicated. The factors needed to obtain a strong RXPS signal are investigated in order to predict the types of systems RXPS will work best for; a balance of element electronegativity and bonding type is found to be important. Additionally, the dependence of RXPS spectra on both varying solvation environment and varying local-covalent bonding is probed. We find that RXPS is a promising fingerprint method for identifying species in solution, due to the spectral shape having a strong dependence on local-covalency but a weak dependence on the solvation environment.
We demonstrate a combined experimental and computational approach to probe the electronic structure and atomic environment of an ionic liquid, based on core level binding energies. The 1-butyl-3-methylimidazolium thiocyanate [C4C1Im][SCN] ionic liquid was studied using ab initio molecular dynamics, and results were compared against previously published and new experimental X-ray photoelectron spectroscopy (XPS) data. The long-held assumption that initial-state effects in XPS dominate the measured binding energies is proven correct, which validates the established premise that the ground state electronic structure of the ionic liquid can be inferred directly from XPS measurements. A regression model based upon site electrostatic potentials and intramolecular bond lengths is shown to account accurately for variations in core-level binding energies within the ionic liquid, demonstrating the important effect of long-range interactions on the core levels and throwing into question the validity of traditional single ion pair ionic liquid calculations for interpreting XPS data.
Non-resonant and resonant photoelectron spectroscopy was used to measure electronic structure descriptors for 60 ionic liquids, allowing the development of design rules to predict the electronic structure descriptors for a further 516 ionic liquids.
Using a combination of liquid-phase X-ray spectroscopy experiments and small-scale calculations we have gained new insights into the speciation of halozincate anions in ionic liquids.
Using a combination of experiments and calculations, we have gained new insights into the nature of anion–cation interactions in ionic liquids (ILs). An X-ray photoelectron spectroscopy (XPS)-derived anion-dependent electrostatic interaction strength scale, determined using XPS core-level binding energies for IL cations, is presented here for 39 different anions, with at least 18 new anions included. Linear correlations of experimental XPS core-level binding energies for IL cations with (a) calculated core binding energies (ab initio molecular dynamics (AIMD) simulations were used to generate high-quality model IL structures followed by single-point density functional theory (DFT) to obtain calculated core binding energies), (b) experimental XPS core-level binding energies for IL anions, and (c) other anion-dependent interaction strength scales led to three main conclusions. First, the effect of different anions on the cation can be related to ground-state interactions. Second, the variations of anion-dependent interactions with the identity of the anion are best rationalized in terms of electrostatic interactions and not occupied valence state/unoccupied valence state interactions or polarizability-driven interactions. Therefore, the XPS-derived anion-dependent interaction strength scale can be explained using a simple electrostatic model based on electrostatic site potentials. Third, anion–probe interactions, irrespective of the identity of the probe, are primarily electrostatic, meaning that our electrostatic interaction strength scale captures some inherent, intrinsic property of anions independent of the probe used to measure the interaction strength scale.
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