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Seymour JM, Gousseva E, Large AI, Clarke CJ, Licence P, Fogarty RM, Duncan DA, Ferrer P, Venturini F, Bennett RA, Palgrave RG, Lovelock KRJ. Experimental measurement and prediction of ionic liquid ionisation energies. Phys Chem Chem Phys 2021; 23:20957-20973. [PMID: 34545382 DOI: 10.1039/d1cp02441h] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Ionic liquid (IL) valence electronic structure provides key descriptors for understanding and predicting IL properties. The ionisation energies of 60 ILs are measured and the most readily ionised valence state of each IL (the highest occupied molecular orbital, HOMO) is identified using a combination of X-ray photoelectron spectroscopy (XPS) and synchrotron resonant XPS. A structurally diverse range of cations and anions were studied. The cation gave rise to the HOMO for nine of the 60 ILs presented here, meaning it is energetically more favourable to remove an electron from the cation than the anion. The influence of the cation on the anion electronic structure (and vice versa) were established; the electrostatic effects are well understood and demonstrated to be consistently predictable. We used this knowledge to make predictions of both ionisation energy and HOMO identity for a further 516 ILs, providing a very valuable dataset for benchmarking electronic structure calculations and enabling the development of models linking experimental valence electronic structure descriptors to other IL properties, e.g. electrochemical stability. Furthermore, we provide design rules for the prediction of the electronic structure of ILs.
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Affiliation(s)
- Jake M Seymour
- Department of Chemistry, University of Reading, Reading, RG6 6AD, UK.
| | | | - Alexander I Large
- Department of Chemistry, University of Reading, Reading, RG6 6AD, UK. .,Diamond Light Source, Didcot, Oxfordshire, OX11 0DE, UK
| | - Coby J Clarke
- School of Chemistry, University of Nottingham, Nottingham, NG7 2RD, UK
| | - Peter Licence
- School of Chemistry, University of Nottingham, Nottingham, NG7 2RD, UK
| | | | | | - Pilar Ferrer
- Diamond Light Source, Didcot, Oxfordshire, OX11 0DE, UK
| | | | - Roger A Bennett
- Department of Chemistry, University of Reading, Reading, RG6 6AD, UK.
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Kuusik I, Kook M, Pärna R, Kisand V. Ionic Liquid Vapors in Vacuum: Possibility to Derive Anodic Stabilities from DFT and UPS. ACS OMEGA 2021; 6:5255-5265. [PMID: 33681566 PMCID: PMC7931180 DOI: 10.1021/acsomega.0c05369] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/03/2020] [Accepted: 01/29/2021] [Indexed: 05/28/2023]
Abstract
Ultraviolet photoelectron spectroscopy (UPS) investigations of several gas-phase ionic liquid (IL) ion pairs have been conducted. [EMIM][OTF], [PYR14][OTF], [EMIM][DCA], [PYR14][DCA], [PYR14][TCM], [PYR14][FSI], [PYR14][PF6], [S222][TFSI], [P4441][TFSI], and [EMMIM][TFSI] vapor UPS spectra are presented for the first time. The experimental low-binding-energy cutoff value (highest occupied molecular orbital, HOMO energy) of the ionic liquid ion pairs, which is of great interest, has been measured. Many studies use calculated gas-phase electronic properties to estimate the liquid-phase electrochemical stability. Hybrid density functional theory (DFT) calculations have been used to interpret the experimental data. The gas-phase photoelectron spectra in conjunction with the theoretical calculations are able to verify most HOMO energies and assign them to the cation or anion. The hybrid M06 functional is shown to offer a very good description of the ionic liquid electronic structure. In some cases, the excellent agreement between the UPS spectra and the M06 calculation validates the conformer found and constitutes as a first indirect experimental determination of ionic liquid ion-pair structure. Comparisons with recent theoretical studies are made, and implications for electrochemical applications are discussed. The new data provide a much-needed reference for future ab initio calculations and support the argument that modeling of IL cations and anions separately is incorrect.
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Affiliation(s)
- Ivar Kuusik
- Institute of Physics, University of Tartu, W. Ostwaldi 1, EE-50411 Tartu, Estonia
| | - Mati Kook
- Institute of Physics, University of Tartu, W. Ostwaldi 1, EE-50411 Tartu, Estonia
| | - Rainer Pärna
- Institute of Physics, University of Tartu, W. Ostwaldi 1, EE-50411 Tartu, Estonia
| | - Vambola Kisand
- Institute of Physics, University of Tartu, W. Ostwaldi 1, EE-50411 Tartu, Estonia
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Kuusik I, Kook M, Pärna R, Kivimäki A, Käämbre T, Reisberg L, Kikas A, Kisand V. The electronic structure of ionic liquids based on the TFSI anion: A gas phase UPS and DFT study. J Mol Liq 2019. [DOI: 10.1016/j.molliq.2019.111580] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
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Kuusik I, Berholts M, Kruusma J, Tõnisoo A, Lust E, Nõmmiste E, Kisand V. Valence electronic structure of [EMIM][B(CN)4]: ion-pair vs. bulk description. RSC Adv 2019; 9:33140-33146. [PMID: 35529163 PMCID: PMC9073151 DOI: 10.1039/c9ra06762k] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2019] [Accepted: 10/10/2019] [Indexed: 11/21/2022] Open
Abstract
The ultraviolet photoelectron spectrum of the [EMIM][B(CN)4] ionic liquid was recorded and analyzed. Together with different ab initio calculation methods, detailed insight into the electronic structure of this simple room temperature ionic liquid is possible. The ion-pair approximation to the liquid electronic structure was not sufficient. Therefore bulk ab initio calculations were performed on a proposed crystal structure. The modelling of bulk electronic spectra is able to explain the experimental electronic structure of the ionic liquid. Most notably, the dispersion corrected PBE calculation (PBE-D3BJ) showed good agreement with the experimental UPS spectrum. The spectra simulated by the B97-D and the BLYP-D3(BJ) functionals were also in agreement with the experimental data. The LDA approximation only provided qualitative agreement while the optB88-vdW and CX-vdW functionals were not good. However, it will be shown that many requirements have to be met in order to accurately describe the electronic structure of this ionic liquid. The ultraviolet photoelectron spectrum of the [EMIM][B(CN)4] ionic liquid was recorded and simulated using different ab initio ion-pair and bulk calculation methods.![]()
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Affiliation(s)
- I. Kuusik
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
| | - M. Berholts
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
- Dept. of Physics and Astronomy
| | - J. Kruusma
- Institute of Chemistry
- University of Tartu
- 50411 Tartu
- Estonia
| | - A. Tõnisoo
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
| | - E. Lust
- Institute of Chemistry
- University of Tartu
- 50411 Tartu
- Estonia
| | - E. Nõmmiste
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
| | - V. Kisand
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
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Kuusik I, Berholts M, Kruusma J, Kisand V, Tõnisoo A, Lust E, Nõmmiste E. Valence electronic structure of [EMIM][BF4] ionic liquid: photoemission and DFT+D study. RSC Adv 2018; 8:30298-30304. [PMID: 35546846 PMCID: PMC9085424 DOI: 10.1039/c8ra05865b] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2018] [Accepted: 08/22/2018] [Indexed: 11/21/2022] Open
Abstract
The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF4] ionic liquid was recorded and analyzed. Together with the gas-phase UPS spectrum of the [EMIM][BF4] vapor and ab initio calculation methods, detailed insight into the electronic structure of this simple ionic liquid is possible. The low binding energy tail in the UPS spectrum is about 7.4 eV, in agreement with previous estimations of the HOMO–LUMO gap of the [EMIM][BF4] ion-pair. The bulk ab initio calculations are able to explain most of the features in the spectrum. However, DFT consistently lacks accuracy in the description of the top of the valence band. The dispersion corrected PBE calculation (PBE-D3) did offer very good agreement with the experimental structure, but the recently-developed vdW-DF functionals C09, optPBE, optB88 and CX were found to offer the best agreement in terms of the electronic structure. The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF4] ionic liquid was recorded and compared to previously measured vapor phase UPS spectrum.![]()
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Affiliation(s)
- I. Kuusik
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
| | - M. Berholts
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
- Dept. of Physics and Astronomy
| | - J. Kruusma
- Institute of Chemistry
- University of Tartu
- 50411 Tartu
- Estonia
| | - V. Kisand
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
| | - A. Tõnisoo
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
| | - E. Lust
- Institute of Chemistry
- University of Tartu
- 50411 Tartu
- Estonia
| | - E. Nõmmiste
- Institute of Physics
- University of Tartu
- 50411 Tartu
- Estonia
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