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.

Control over the average length of single-walled carbon nanotubes (SWNTs) in suspension is of critical importance to characterizing and developing flexible, transparent thin films that use percolative transport to achieve reproducibility in electronic properties. This paper demonstrates how the average length of SWNTs in aqueous suspensions can be controlled by the conditions used to form (sonication) and purify (low-G centrifugation) the dispersions. The effect of ultrasonic probe sonication, which was used to disperse SWNT bundles into suspension, on the length and extent of defects on the nanotubes was investigated via atomic force microscopy (AFM) and confocal Raman spectroscopy, respectively. Quantitative information about the suspension concentration and the effect of sonication power on unbundling the SWNTs was obtained via UV−vis and near-IR spectroscopy, respectively. To obtain a clear understanding of the effect of sonication power on SWNT suspensions, repeated low-G centrifugation cycles were used to remove impurities such as bundles of SWNTs, amorphous carbon, and catalyst nanoparticles. This nonoxidizing purification method, which was performed prior to all analyses, allows direct determination of the effect of sonication power on defect formation.

The entrance channel complex in the exothermic OH + CH → HO + CH reaction has been isolated in helium nanodroplets following the sequential pick-up of the hydroxyl radical and methane. The a-type OH stretching band was probed with infrared depletion spectroscopy, revealing a spectrum qualitatively similar to that previously reported in the gas phase, but with additional substructure that is due to the different internal rotation states of methane (j = 0, 1, or 2) in the complex. We fit the spectra by assuming the rotational constants of the complex are the same for all internal rotation states; however, subband origins are found to decrease with increasing j. Measurements of deuterated complexes have also been made (OD-CH, OH-CD, and OD-CD), the relative linewidths of which provide information about the flow of vibrational energy in the complexes; vibrational lifetime broadening is prominent for OH-CH and OD-CD, for which the excited OX stretching state has a nearby CY stretching fundamental (X, Y = H or D).