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

Raman spectra in the range of the totally symmetric stretching mode of the [PF₆]^- anion, ν_s(PF₆), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [C_nC 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 ν_s(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, τ_c, as a function of density by Raman band shape analysis of the ν_s(PF₆) mode of [C₆C₁im][PF ₆] and [C₈C₁im][PF₆].