Physical Properties and Low-Frequency Polarizability Anisotropy and Dipole Responses of Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids with Pentyl, Ethoxyethyl, or 2-(Ethylthio)ethyl Group

Masatoshi Ando; Kaoru Ohta; Tateki Ishida; Ryohei Koido; Hideaki Shirota

doi:10.1021/acs.jpcb.2c07466

Physical Properties and Low-Frequency Polarizability Anisotropy and Dipole Responses of Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids with Pentyl, Ethoxyethyl, or 2-(Ethylthio)ethyl Group

J Phys Chem B. 2023 Jan 19;127(2):542-556. doi: 10.1021/acs.jpcb.2c07466. Epub 2023 Jan 5.

Authors

Masatoshi Ando¹, Kaoru Ohta², Tateki Ishida³, Ryohei Koido¹, Hideaki Shirota¹

Affiliations

¹ Department of Chemistry, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522, Japan.
² Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan.
³ Department of Theoretical and Computational Molecular Science, Institute for Molecular Science and Research Center for Computational Science, 38 Nishigonaka, Myodaiji, Okazaki 444-8585, Japan.

PMID: 36602430
DOI: 10.1021/acs.jpcb.2c07466

Abstract

This study compared the physical properties, e.g., glass transition temperature, melting point, viscosity, density, surface tension, and electrical conductivity, and the low-frequency spectra under 200 cm^-1 of three synthesized ionic liquids (ILs), triethylpentylphosphonium bis(fluorosulfonyl)amide ([P₂₂₂₅][NF₂]), ethoxyethyltriethylphosphonium bis(fluorosulfonyl)amide ([P_222(2O2)][NF₂]), and triethyl[2-(ethylthio)ethyl]phosphonium bis(fluorosulfonyl)amide ([P_222(2S2)][NF₂]), at various temperatures using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and terahertz time-domain spectroscopy (THz-TDS). The [P_222(2S2)][NF₂] had the highest viscosity and glass transition temperature, whereas the [P_222(2O2)][NF₂] had the lowest. Among the three ILs, the [P_222(2S2)][NF₂] had the highest density and surface tension, and the [P_222(2O2)][NF₂] had the highest electrical conductivity. The RIKES and THz-TDS spectral line shapes for the three ILs varied significantly. For the [P₂₂₂₅][NF₂], molecular dynamics simulations successfully reproduced the line shapes of the experimental spectra and indicated that the RIKES spectrum was mainly due to the cation and cross-term and their rotational motions, whereas the THz-TDS spectrum was mainly due to the anion and its translational motion. This shows that it is desirable to utilize both fs-RIKES and THz-TDS methods to reveal molecular motions at the low-frequency domain. The [P_222(2S2)][NF₂] had higher frequency peaks and broader bands in the low-frequency spectra via fs-RIKES and THz-TDS than those for the [P₂₂₂₅][NF₂] and [P_222(2O2)][NF₂].