Ultrasonic Relaxation Spectra for Pyrrolidinium Bis(trifluoromethylsulfonyl)imides: A Comparison with Imidazolium Bis(trifluoromethylsulfonyl)imides

Ultrasound absorption spectra within the frequency range 10–300 MHz were determined for 1-propyl- and 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imides at ambient pressure and at temperatures in the ranges 293.15–313.15 and 293.15–323.15 K, respectively. For both compounds, a single Debye model (relaxation times between 0.451 and 0.778 ns) thoroughly describes the observed ultrasound absorption spectra in the investigated ranges. The spectra resemble those observed for imidazolium-based ionic liquids with the same anion. The ultrasound relaxation is dependent on the alkyl chain length of pyrrolidinium ring. In comparison to adequate imidazolium-based bis­(trifluoromethylsulfonyl)­imides, the relaxation in pyrrolidinium-based bis­(trifluoromethylsulfonyl)­imides is stronger; the pyrrolidinium cation causes clearly greater absorption than the imidazolium cation. Also, estimated ultrasound velocity dispersion is stronger in the case of pyrrolidinium imides in comparison to imidazolium imides. In turn, comparison of the ultrasonic data and literature data for the dielectric spectra exemplified for the 1-butyl- side chain in the cation indicates strong coupling in the case of imidazolium ring and weak coupling in the case of pyrrolidinium ring. The effect of absorption on the speed of sound is also discussed.