Vibrational frequency shift of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids under high pressure

The Raman band belonging to the totally symmetric stretching mode of the [BE4](-) anion, v(s)(BF4), in 1-alkyl-3-methylimidazolium ionic liquids, [C(n)C(1)im][BF4], n = 4, 6, and 8, has been measured under high pressure using a diamond anvil cell. The pressure dependence of v(s)(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 v(s)(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 [C(n)C(1)im][PF6] ionic liquids, for which the calculation considered only the repulsive short-range forces, in the case of [C(n)C(1)im][BF4] the calculation indicates the role played by the attractive forces on the v(s)(BF4) frequency shift. (C) 2019 Elsevier B.V. All rights reserved.