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 is vaporized at 420 K, and the ion-pair constituents are entrained in a beam of liquid He nanodroplets and cooled to 0.4 K. The vapor pressure is optimized such that each He droplet picks up a single ion-pair from the gas...

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Bibliographic Details
Main Authors: Obi, Emmanuel I, Leavitt, Christopher M, Raston, Paul L, Moradi, Christopher P, Flynn, Steven D, Boatz, Jerry A, Chambreau, Steven D, Douberly, Gary E
Format: Report
Language:English
Subjects:
ALLOCATIONS
ANIONS
BANDS(STRIPS)
CATIONS
CHEMICAL REACTIONS
COMPUTATIONS
CONSISTENCY
DIPOLE MOMENTS
DIPOLES
ELECTRIC MOMENTS
FREE ENERGY
HYDROGEN BONDS
INFRARED SPECTRA
INFRARED SPECTROSCOPY
Inorganic Chemistry
IONS
LOW ENERGY
Optics
Physical Chemistry
RESONANCE
SPECTROSCOPY
VAPOR PHASES
VAPOR PRESSURE
WEIGHTING FUNCTIONS
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Summary:The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is vaporized at 420 K, and the ion-pair constituents are entrained in a beam of liquid He nanodroplets and cooled to 0.4 K. The vapor pressure is optimized such that each He droplet picks 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 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. The original document contains color images. Pub. in Journal of Physical Chemistry, p1-30, 2013.