To clarify bulk spectroscopic evidence that cooperative hydrogen bonding interactions between like-charges can compete with Coulomb repulsions in hydroxyl-functionalized ionic liquids, we report the vibrational spectra of a variety of isolated cations as well as clusters consisting of two cations and one NTf2- in the cationic (2,1) clusters. These spectra were obtained by the messenger (N2) tagging method applied to mass-selected, cryogenically-cooled (~35 K) ions in the gas phase. To specifically address a range of interaction strengths between the ionic constituents, we surveyed a series of cations selected to reveal the interplay between H-bonding of the cation to the anion and H-bonding between the cationic moieties. The band patterns were analyzed with harmonic calculations for various structural isomers identified in a search for the likely structural motifs involving interactions between the ionic constituents. The formation of several (up to three) distinct isomers was established for the 1-(2-Hydroxyethyl)-3-methylimidazolium (HEMIm) complex using IR−IR double resonance spectroscopy, which directly revealed the delicate competition between the electroscopic bond between the counter ions and the attractive, H-bonding interaction between the cations.

Elucidating the microscopic mechanics of attractive cation-cation interactions in hydroxyl-functionalized ionic liquids by isomer-selective vibrational spectroscopy of the N2-tagged, gas phase (M+)2NTf2- ternary complexes

255th National Meeting (2018)

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