Atomic charges of sulfur in ionic liquids: experiments and calculations

Abstract

Experimental near edge X-ray absorption fine structure (NEXAFS) spectra, X-ray photoelectron (XP) spectra and Auger electron spectra are reported for sulfur in ionic liquids (ILs) with a range of chemical structures. These values provide experimental measures of the atomic charge in each IL and enable the evaluation of the suitability of NEXAFS spectroscopy and XPS for probing the relative atomic charge of sulfur. In addition, we use Auger electron spectroscopy to show that when XPS binding energies differ by less than 0.5 eV, conclusions on atomic charge should be treated with caution. Our experimental data provides a benchmark for calculations of the atomic charge of sulfur obtained using different methods. Atomic charges were computed for lone ions and ion pairs, both in the gas phase (GP) and in a solvation model (SMD), with a wide range of ion pair conformers considered. Three methods were used to compute the atomic charges: charges from the electrostatic potential using a grid based method (ChelpG), natural bond orbital (NBO) population analysis and Bader's atoms in molecules (AIM) approach. By comparing the experimental and calculated measures of the atomic charge of sulfur, we provide an order for the sulfur atoms, ranging from the most negative to the most positive atomic charge. Furthermore, we show that both ChelpG and NBO are reasonable methods for calculating the atomic charge of sulfur in ILs, based on the agreement with both the XPS and NEXAFS spectroscopy results. However, the atomic charges of sulfur derived from ChelpG are found to display significant, non-physical conformational dependence. Only small differences in individual atomic charge of sulfur were observed between lone ion (GP) and ion pair IL(SMD) model systems, indicating that ion-ion interactions do not strongly influence individual atomic charges.

Calculation of core‐level electron spectra of ionic liquids

“…Recently it was proposed that the BEs obtained from fitting the experimental XPS spectra for ILs correlate with the calculated atomic charges. [ 32,33 ] Figure 6 demonstrates a modest correlation between the BE approximated by the first and third terms of Equation ) (BE = V [ q i ]) and the calculated relative ΔKS BE values. The relation improves significantly by considering the effect of the electrostatic potential of neighboring atoms given by the second term in Equation ) (BE = V [ q i , q j ]).…”

“…[ 31 ] Fogarty et al found such correlation for the S 1s electrons ( R 2 = 0.98) and N 1s electrons ( R 2 = 0.94) between the experimental BEs and the computed atomic charges. [ 32,33 ] Kruusma et al calculated C 1s electrons' Kohn–Sham (KS) orbital energies and used their values for fitting the experimental spectra. [ 17 ] Similarly, Reinmöller et al used the KS orbital energies to calculate the BEs for the XPS spectra.…”

Resolving X-ray photoelectron spectra of ionic liquids with difference spectroscopy

“…However, due to the lack of standard procedures, different C 1s fitting models have been developed for even the most basic IL chemical structures. For example, the C 1s region of imidazolium ILs have been fitted with 2-3 component models, which broadly account for polar and non-polar regions, 32 or more complex fittings with 3+ components. 21,33 While the latter can potentially provide more information, simpler models are a conservative approach aimed at minimising over interpretation of XP spectra.…”

“…shifts < 0.5 eV in IL XP spectra in terms of atomic charges. 32,37 Despite this, correlations between core-electron B.E.s and physicochemical properties (e.g. Kamlettaft parameters) 21,22,31 support the drive for accurate peak fittings, regardless of the physical interpretation of the data.…”

Computational Spectroscopy of Ionic Liquids for Bulk Structure Elucidation

“…Lovelock and co‐workers reported a systematic investigation of several ILs by means of NEXAFS (near‐edge X‐ray absorption fine structure) spectroscopy and XPS (X‐ray photoelectron spectroscopy) . In these experiments, the core electrons of a given atom are excited to an unoccupied molecular orbital by X‐ray radiation.…”