Low Frequency Vibrational Modes of Room Temperature Ionic Liquids

The vibrational spectra of four ionic liquids, sharing the same imidazolium cation but containing different anions-[bmim][NO3], [bmirn][BF4], [bmim][PF6], and [bmim][NTf2]-have been obtained using normal-mode analysis within the harmonic approximation and from velocity autocorrelation fimctions from a molecular dynamics trajectory generated using empirical force fields. The vibrational density of states obtained from the two methods agree well. The low frequency modes (<100 cm(-1)) exhibit a red shift with an increase in the anion size. Deuteration of the ring hydrogens leads to a negligible change in this region of the spectrum. The participation ratio of low frequency modes is large, implying that they are not localized to a few atoms. The low frequency band arises primarily from short-range interionic interactions, and the exact peak position is modulated by the cation-anion hydrogen bond strength. Results obtained from these force-field-based calculations are confirmed by ab initio molecular dynamics simulations of crystalline [bmim][PF6].