Atom substitution effects of [XF6]- in ionic liquids. 1. Experimental study

We have investigated the interionic vibrational dynamics of 1-butyl-3-methylimidazolium cation ([BMIm]+) based ionic liquids with the anions of [PF6]-, [AsF6]-, and [SbF6]- as well as the static physical properties, such as shear viscosity and liquid density. Shear viscosity for the ionic liquids becomes lower with the heavier atom anion: [BMIm][PF6]>[BMIm][AsF6]>[BMIm][SbF6]. This tendency for heavy atom substitution for the anion results from weaker interionic interaction caused by larger anion volume. Femtosecond optically heterodyne-detected Raman-induced Kerr effect spectroscopy has been used to observe the interionic vibrational dynamics of ionic liquids. The interionic vibration in the frequency region of less than 50 cm(-1) clearly shows the heavy atom substitution effect; that is, the heavy atom substitution of [XF6]- critically affects the interaction-induced motion. The forthcoming paper will further provide the molecular-level insights of the heavy atom substitution effect of the [XF6]- anion on the interionic dynamics and interaction for the three ionic liquids by a molecular dynamics simulation approach.