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

• Published in: The journal of physical chemistry. B 2023-01, Vol.127 (2), p.542-556
• Main Authors: Ando, Masatoshi, Ohta, Kaoru, Ishida, Tateki, Koido, Ryohei, Shirota, Hideaki
• Format: Article
• Language: English
• Subjects: B: Liquids; Chemical and Dynamical Processes in Solution
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.2c07466

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 ([P222(2O2)]­[NF2]), and triethyl­[2-(ethylthio)­ethyl]­phosphonium bis­(fluorosulfonyl)­amide ([P222(2S2)]­[NF2]), at various temperatures using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and terahertz time-domain spectroscopy (THz-TDS). The [P222(2S2)]­[NF2] had the highest viscosity and glass transition temperature, whereas the [P222(2O2)]­[NF2] had the lowest. Among the three ILs, the [P222(2S2)]­[NF2] had the highest density and surface tension, and the [P222(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 [P222(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 [P222(2O2)]­[NF2].