Methyl quantum tunneling in ionic liquid [DMIm][TFSI] facilitated by Bis(trifluoromethane)sulfonimide lithium salt

Abstract

We probe, for the first time, quantum tunneling in the methyl groups of the ionic liquid [DMIm][TFSI] facilitated by the presence of Bis(trifluoromethane)sulfonimide lithium salt. The observation of tunneling is made possible by crystallization, rather than vitrification, of [DMIm][TFSI] at low temperature. Neutron scattering measurements detect quantum tunneling excitations at ~27 μeV at temperatures below 30 K in the presence of LiTFSI at a concentration of 1 mol/kg, but not in salt-free [DMIm][TFSI]. This indicates that the methyl rotational potential barrier is reduced by the presence of LiTFSI, thus bringing the tunneling excitations into the measurable range. The salt-induced reduction of the rotational barrier is corroborated by quasi-elastic scattering data associated with stochastic re-orientation of methyl groups measured between 40 and 60 K.