Which ab initio calculation methods describe the charge transfer in ionic liquid vapors? Case study on EMIM-OTF...

Time-of-flight mass-spectrometry (TOF-MS) measurements of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTF]) vapors have been conducted in order to gain insight into the VUV (vacuum ultraviolet) absorption thresholds. The experimental data were interpreted with hybrid density-functional theory (DFT), time-dependent DFT (TD-DFT), and several flavours of many-body perturbation theory (MBPT): second-order MBPT, Hedin's GW and Bethe-Salpeter equation (BSE) calculations. To our knowledge, this is the first comprehensive comparison of DFT functionals for an ionic liquid (IL) vapor. More than 60 exchange-correlation approximations, including many range-separated hybrids (RSHs) and optimally tuned variants, were assessed. The theoretical methods predicted a wide range of energies; only the experimental TOF-MS data allowed the truly predictive approaches to be identified. The experimental spectra reveal charge transfer (CT) states at an optical gap of 6.75 eV and a maximum ion yield signal at 8.3 eV. Combining the experiment and theory enabled the re-examination of the ion-pair geometry and the reconstruction of the frontier orbitals of [EMIM][OTF]. The HOMO and LUMO energies were found to be -9.05 eV and -2.30 eV, respectively; energies for HOMO-1 and LUMO+1 were also determined. Although a ΔSCF calculation with the non-hybrid M11-L functional gives a reasonable ionization threshold, ≥50% exact Hartree-Fock (HF) exchange is required to correctly describe both the optical gap and the ultraviolet photoemission spectrum (UPS). The RCAM-B3LYP functional and the Bartlett's QTP family of functionals showed good performance in predicting the CT excitations from TD-DFT calculations. No method was able to predict the proposed LUMO energy, though the M08-SO functional was closest overall. Within the limitations of the present work, the global hybrid PBE-2X functional demonstrates the most consistent overall performance. Its range-separated hybrid (RSH) analogue CAM-PBE50* also performed well for the calculation of CT excitations.