Helium Nanodroplet Isolation and Infrared Spectroscopy of the Isolated Ion-Pair 1‑Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

The ionic liquid 1-ethyl-3-methylimidazolium
bis­(trifluoromethylsulfonyl)­imide
was vaporized at 420 K, and the ion-pair constituents were entrained
in a beam of liquid He nanodroplets and cooled to 0.4 K. The vapor
pressure was optimized such that each He droplet picked up a single
ion-pair from the gas phase. Infrared spectroscopy in the CH stretch
region reveals bands that are assigned to intact ion-pairs on the
basis of comparisons to ab initio harmonic frequency computations
of 23 low energy isomers. The He droplet spectrum is consistent with
a weighted sum of the computed harmonic spectra, in which the weights
are determined from ab initio computations of the relative free energies
at 420 K. Anharmonic resonance polyads in the CH stretch region are
treated explicitly, which improves the agreement between the experiment
and computed spectra for ion-pairs. For isomers having a strong cation···anion
hydrogen bonding interaction, the imidazolium C(2)-H stretch
fundamental is shifted to lower energy and into resonance with the
overtones and combination bands of the imidazolium ring stretching
modes, resulting in a spectral complexity in the CH stretch region
that is fully resolved in the He droplet spectrum. The assignment
of the infrared spectrum to ion-pairs is confirmed through polarization
spectroscopy measurements that reveal the permanent electric dipole
moment of the He-solvated species to be 11 ± 2 D. The computed
permanent electric dipole moments for the low energy isomers of the
[emim+]­[Tf2N–] ion-pairs fall
in the range 9–13 D, whereas
the computed dipole moments of decomposition products of the ionic
liquid are less than 4.3 D.