Pressure effect on vibrational frequency and dephasing of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids

• Published in: The Journal of chemical physics 2013-08, Vol.139 (5), p.054510-054510
• Main Authors: Pison, L, Costa Gomes, M F, Pádua, A A H, Andrault, D, Norman, S, Hardacre, C, Ribeiro, M C C
• Format: Article
• Language: English
• Subjects: Amplitudes; Earth Sciences; Imidazoles - chemistry; Ionic Liquids - chemistry; Petrography; Pressure; Sciences of the Universe; Spectrum Analysis, Raman; Vibration
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.4817403

Raman spectra in the range of the totally symmetric stretching mode of the [PF6](-) anion, νs(PF6), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [CnC1im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C6C1im][PF6] and [C8C1im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C4C1im][PF6], which crystallizes above ~0.5 GPa. Equations of state based either on a group contribution model or Carnahan-Starling-van der Waals model have been used to estimate the densities of the ionic liquids at high pressures. The shifts of the vibrational frequency of νs(PF6) with density observed in [C6C1im][PF6] and in [C8C1im][PF6] have been calculated by a hard-sphere model of a pseudo-diatomic solute under short-range repulsive interactions with the neighboring particles. The stochastic model of Kubo for vibrational dephasing has been used to obtain the amplitude of vibrational frequency fluctuation, , and the relaxation time of frequency fluctuation, τc, as a function of density by Raman band shape analysis of the νs(PF6) mode of [C6C1im][PF6] and [C8C1im][PF6].