title: 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][NO(3)], [bmim][BF(4)], [bmim][PF(6)], and [bmim][NTf(2)]-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][PF(6)].
authors.affiliation: Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India.
authors.forename: S S
authors.initials: SS
authors.lastname: Sarangi
authors.forename: S K
authors.initials: SK
authors.lastname: Reddy
authors.forename: S
authors.initials: S
authors.lastname: Balasubramanian