Session 7

Intermolecular Vibrational Band of Deep Eutectic Solvents Composed of Lithium Salt and Amide: Composition Dependence

Hideaki Shirota
on  Wed, 11:30 for  20min
M. Koyakkat, H. Shirota
1. 1-33 Yayoi, Inage-ku, Chiba 263-8522, Department of Chemistry, Chiba University
shirota@faculty.chiba-u.jp

Abstract

The eutectic phenomena are recognized in our surroundings, such as solder and anti-icing agents. When a mixture composed of more than two components shows a huge reduction in melting point compared to the neat components and it becomes the liquid state at ambient temperatures, it is referred to as a deep eutectic solvent (DES). The great advantages of DES include its easy preparation, often just mixing (or sometimes with mild heat), and environmentally friendly nature, when the components are appropriately selected. DES is thus getting more attention recently [1,2].

Typical DESs are composed of a pair of hydrogen-bonding donor and acceptor (often a salt). Because the intermolecular interaction in DES is strong, the viscosity is high. Further, the intermolecular interaction in DESs is complicated because they involve cations, anions, and hydrogen-bonding donors. An effective approach to understand the microscopic intermolecular interaction in liquids/solutions is to examine the intermolecular vibrational band, which is located in the low-frequency region < ~150 cm-1 [3,4]. Herein, we report the intermolecular vibrational behaviour of (i) lithium bis(trifluoromethylsulfonyl)amide/organic amide systems [5] and (ii) lithium salt/acetamide systems [6].

In the study of lithium bis(trifluoromethylsulfonyl)amide/organic amide systems [5], we compared five organic amides (acetamide, acetamide, propanamide, N-methylacetamide, butyramide, and urea). The line shape and vibrational frequency of the low-frequency spectrum depend on the host amide. The peak frequency and the first moment of the intermolecular vibrational bands of the acetamide- and urea-based DESs were in a higher-frequency region compared to the other three DESs, which indicates stronger microscopic interactions in the former case.

In the study of lithium salt/acetamide systems [6], we investigated the anion dependence of the intermolecular vibrational dynamics of DESs composed of five lithium salts and acetamide. The line shape and first moment of the intermolecular vibrational band strongly depended on the anion species. The order of the first moments indicates that the microscopic intermolecular interactions are stronger in the DESs of lithium salts with nitrate [NO3]– and trifluoromethanesulfonate [OTf]– than in those of lithium salts with perchlorate [ClO4]–, bis(fluorosulfonyl)amide [NF2]–, and bis(trifluoromethylsufonyl)amide [NTf2]–. Like simple aprotic molecular liquids, a correlation of the first moment of the low-frequency band and the bulk parameter (the square root of surface tension divided by density) in the present DES systems is confirmed, except for Li[OTf]/acetamide system. Compared with the lithium bis(trifluoromethylsulfonyl)amide/organic amide systems, the anion of lithium salt is more sensitive to the intermolecular vibration and bulk liquid properties than the host organic amide.

References

[1] E.L. Smith, A.P. Abbott, K.S. Ryder, Chem. Rev. 114, 11060 (2014).

[2] B. B. Hansen, S. Spittle, B. Chen, D. Poe, Y. Zhang, J. M. Klein, A. Horton, L. Adhikari, T. Zelovich, B. W. Doherty, B. Gurkan, E. J. Maginn, A. Ragauskas, M. Dadmun, T. A. Zawodzinski, G. A. Baker, M. E. Tuckerman, R. F. Savinell, and J. R. Sangoro, Chem. Rev. 121, 1232 (2021).

[3] D. McMorrow, W. T. Lotshaw, and G. A. Kenneywallace, IEEE J. Quantum Electron. 24, 443 (1988).

[4] N. A. Smith and S. R. Meech, Int. Rev. Phys. Chem. 21, 75 (2002)

[5] M. Koyakkat and H. Shirota, J. Phys. Chem. B 129, 4023 (2025)

[6] M. Koyakkat and H. Shirota, J. Chem. Phys. 164, 144507 (2026).