IR and UV Spectroscopy of Vapor-Phase Jet-Cooled Ionic Liquid [emim]⁺[Tf₂N]⁻: Ion Pair Structure and Photodissociation Dynamics

Abstract

Small gas-phase clusters (ion pairs) of the ionic liquid [emim](+)[Tf2N](-) have been generated in a supersonic expansion. Clusters are investigated via UV photofragmentation and time-of-flight mass spectrometry. Spectra between 42,000 and 45,000 cm(-1) reveal dynamical branching between direct dissociation of the ion pair to the cation and anion and to radical species. The IR spectrum between 2800 and 3200 cm(-1) was measured by action spectroscopy. Multiple conformations of the ion pair are found to be present in the molecular beam, leading to broad spectral features, further complicated by hydrogen bonding and Fermi resonances. The measured and theoretical spectra compare well, and the jet-cooled ion pair structures present in the molecular beam are strongly hydrogen bonded "stacked" conformers.

Imidazolium Salt Ion Pairs in Solution

“…[76,77] Recent IR spectroscopic measurements of the vapour-phase jet-cooled ionic liquid EMI·NTf 2 has shed some light into the assignment of the vibrational bands within this frequency range. [78] For isolated ion pairs, Cooper et al could assign the highest frequency peaks to the symmetric and anti-symmetric stretches of the C(4)ÀH and C(5)ÀH bonds at the imidazolium ring at 3155 and 3185 cm À1 , respectively (see Figure 11).…”

“…The experimental spectrum best matches the calculated spectra of family II. Reprinted from reference [78] pairs with an effective dipole moment of 22.9 D have been identified at low IL mole fractions. Furthermore, the association constant for the EMI·EtSO 4 ion pair has been found to be lower than for the BMI·BF 4 pair.…”

Understanding the ionic liquid [NC4111][NTf2] from individual building blocks: an IR-spectroscopic study

“…Fig. [36][37][38][39] A recent comparison of IR and UV/VIS bulk spectra of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with DFT and MP2 calculations for the gas phase and in a continuum solvent yielded surprisingly good agreement between gas phase predictions and the experimental spectra of the bulk material. The observed bands appear in well-separated, localized regions of the spectrum, with those arising from the (NTf 2 ) À anion colored blue while those from the (NC 4111 ) + cation colored red, with the various groups primarily responsible for transitions indicated at the top.…”

Structure of protic (HCnImNTf2, n=0–12) and aprotic (C1CnImNTf2, n=1–12) imidazolium ionic liquids: A vibrational spectroscopic study

“…Furthermore, its overall intensity is decreased compared to the C 4,5 -H. We have to note here that the assignment of this band is a matter of debate, since it is well established that there is Fermi resonance with the ring stretching vibrational mode. This make the interpretation of this spectral area quite complicated, as we already mentioned in a recent publication and we will not offer any new discussion here [36][37][38][39][40][41][42][43]. No more shifts are observed with increasing n from 2 to 12.…”