Low frequency vibrational modes of room temperature ionic liquids

Abstract

The vibrational spectra of four ionic liquids, sharing the same imidazolium cation but containing different anions - [bmim][NO₃], [bmim][BF₄], [bmim][PF₆], and [bmim][NTf₂] - have been obtained using normal-mode analysis within the harmonic approximation and from velocity autocorrelation functions 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].