Vibrational frequency shift of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids under high pressure
Abstract The Raman band belonging to the totally symmetric stretching mode of the [BF4]− anion, νs(BF4), in 1-alkyl-3-methylimidazolium ionic liquids, [CnC1im][BF4], n = 4, 6, and 8, has been measured under high pressure using a diamond anvil cell. The pressure dependence of νs(BF4) frequency up to 2.0 GPa is converted to a function of density using high pressure extrapolations of equations of states as discussed in previous works. The density induced shift of the νs(BF4) frequency was calculated by a perturbed hard-sphere model by considering the probe oscillator as a pseudo-diatomic molecule. In contrast to previous findings for the stretching mode of [PF6]− in [CnC1im][PF6] ionic liquids, for which the calculation considered only the repulsive short-range forces, in the case of [CnC1im][BF4] the calculation indicates the role played by the attractive forces on the νs(BF4) frequency shift.