Frontier orbitals and quasiparticle energy levels in ionic liquids

Abstract

Ionic liquids play an important role in many technological applications and a detailed understanding of their frontier molecular orbitals is required to optimize interfacial barriers, reactivity and stability with respect to electron injection and removal. In this work, we calculate quasiparticle energy levels of ionic liquids using first-principles many-body perturbation theory within the GW approximation and compare our results to various mean-field approaches, including semilocal and hybrid density-functional theory and Hartree–Fock. We find that the mean-field results depend qualitatively and quantitatively on the treatment of exchange–correlation effects, while GW calculations produce results that are in excellent agreement with experimental photoelectron spectra of gas phase ion pairs and ionic liquids. These results establish the GW approach as a valuable tool for understanding the electronic structures of ionic liquids.

Ionic Liquid Vapors in Vacuum: Possibility to Derive Anodic Stabilities from DFT and UPS

“…The correct cation π-states emerge as the top of the valence band in bulk calculations and in some hybrid ion-pair calculations. This has already been discussed previously. − The (self-consistent) GW calculations are able to conclusively solve this issue and confirm that the HOMO–LUMO gap of [EMIM]­[BF 4 ] is larger than 9 eV …”

“…21−23 The (self-consistent) GW calculations are able to conclusively solve this issue and confirm that the HOMO−LUMO gap of [EMIM][BF 4 ] is larger than 9 eV. 43 In their gas-phase study of [EMIM][BF 4 ], Kuusik et al showed that MP2 is able to describe only two out of the three distinct outer valence band peaks (peaks A−C). 21 Thus, it seems reasonable to assign the peaks B and C (see ref 21) to [EMIM][BF 4 ] and the peak A (A1 and A2) should no longer be interpreted as the HOMO state of the ion pair.…”

Experimental measurement and prediction of ionic liquid ionisation energies

“…25 Comparisons exist between liquid phase experimental XP/UP spectra for ILs and calculated data, but the structural range of ILs studied is limited. 24, 49, 64-70, 72-76, 79 Calculations showed the HOMO came from the cation for [C n C 1 Im][BF 4 ] (n = 1, 2, 4), 24,[80][81][82] 24,81,82 73,83 , which matched well to experimental HOMO identification, 48,49,[74][75][76] demonstrating that for this limited range of ILs the calculations captured the solvation of the cation and anion acceptably. E i (IL) values have been calculated, e.g.…”

“…E i (IL) = 9.4 eV from this work for [C 8 C 1 Im][BF 4 ], an excellent match to the E i (IL) value derived from data in reference107 . Im][BF 4 ] from reference49 was significantly lower than E th (IL) = 7.8 eV for [C 4 C 1 Im][BF 4 ] from reference 50 and E th (IL) = 8.3 eV for [C 8 C 1 Im][BF 4 ] from our work, strongly suggesting a problem with the charge referencing for the data in reference49 .Using gas phase UPS of neutral ion pairs, E th (ion pair) values were measured by two groups, Leone and co-workers[15][16][17][18] and Kuusik and co-workers[19][20][21][22][23][24][25] . Most of these E th (ion pair) values were for a combination of imidazolium cations and an imide anion, e.g.…”

“…14 Almost all E i values for ILs, E i (IL), have been measured on vaporised gas phase neutral ion pairs. [15][16][17][18][19][20][21][22][23][24][25][26] Whilst measuring E i is relatively facile in the gas phase, a major problem is that most ILs are very tricky to vaporise without significant thermal decomposition occurring/dominating, meaning many IL ion pairs cannot be easily studied in the vapour phase; [27][28][29][30][31][32] furthermore, vapour phase ion pairs do not have the complete solvation environment of the liquid phase. A major hurdle for measuring reliable, reproducible, and comparable binding energies (E B ) 14 and E i for liquid phase ILs is dealing with sample charging during XPS measurements, which is not understood.…”