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

L. Pison, M. F. Costa Gomes, A. A H Pádua, D. Andrault, S. Norman, C. Hardacre, M. C C Ribeiro

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    Abstract

    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 [CnC 1im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C6C1im][PF 6] and [C8C1im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C4C 1im][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 [C 6C1im][PF6] and in [C8C 1im][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, Δω2, 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][PF 6] and [C8C1im][PF6].

    Original languageEnglish
    Article number054510
    JournalJournal of Chemical Physics
    Volume139
    Issue number5
    DOIs
    Publication statusPublished - 7 Aug 2013

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