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.