Conclusions
In this work, we have validated the applicability of density functional
theory (DFT) methods to the prediction of the X-ray photoelectron
spectra (XPS) of ionic liquids. On the example of data for forty ion
pairs, we demonstrated how the core-level electron binding energy (BE)
could be calculated and used to plot theoretical spectra at a low
computational cost. The delta Kohn–Sham (ΔKS) method provides the most
reliable results in comparisson to the experimental spectra. Negative 1s
Kohn–Sham orbital energies show a strong correlation with the
calculated ΔKS BE values. However, the sequence of orbital energies
differs from the ΔKS BEs data. The correlation of the calculated atomic
charges with the ΔKS BE values is notably weaker. Nonetheless, that
correlation can be improved by accounting for the electrostatic
potential due to the surrounding atoms in the ion pair. Thus, the XPS
spectra can be predicted based on the DFT calculations using three
different approaches. The choice between them implies a compromise
between the computational cost and accuracy. Besides being more
accurate, the ΔKS method is also more resource-demanding and less
accessible than the other two methods. The use of 1s Kohn–Sham orbital
energies and atomic charges are justified when qualitative aspects of
X-ray photoelectron spectra are of higher interest than the absolute
binding energy values or when the available computational power is
limited.