title: Size matters! On the way to ionic liquid systems without ion pairing.
abstract: Several, partly new, ionic liquids (ILs) containing imidazolium and ammonium cations as well as the medium-sized [NTf2 ](-) (0.230 nm(3) ; Tf=CF3 SO3 (-) ) and the large [Al(hfip)4 ](-) (0.581 nm(3) ; hfip=OC(H)(CF3 )2 ) anions were synthesized and characterized. Their temperature-dependent viscosities and conductivities between 25 and 80 °C showed typical Vogel-Fulcher-Tammann (VFT) behavior. Ion-specific self-diffusion constants were measured at room temperature by pulsed-gradient stimulated-echo (PGSTE) NMR experiments. In general, self-diffusion constants of both cations and anions in [Al(hfip)4 ](-) -based ILs were higher than in [NTf2 ](-) -based ILs. Ionicities were calculated from self-diffusion constants and measured bulk conductivities, and showed that [Al(hfip)4 ](-) -based ILs yield higher ionicities than their [NTf2 ](-) analogues, the former of which reach values of virtually 100 % in some cases.From these observations it was concluded that [Al(hfip)4 ](-) -based ILs come close to systems without any interactions, and this hypothesis is underlined with a Hirshfeld analysis. Additionally, a robust, modified Marcus theory quantitatively accounted for the differences between the two anions and yielded a minimum of the activation energy for ion movement at an anion diameter of slightly greater than 1 nm, which fits almost perfectly the size of [Al(hfip)4 ](-) . Shallow Coulomb potential wells are responsible for the high mobility of ILs with such anions.
authors.affiliation: Albert-Ludwigs-Universität Freiburg, Institut für Anorganische und Analytische Chemie, Albertstrasse 21, 79104 Freiburg (Germany); Freiburger Materialforschungszentrum, Stefan-Meier-Strasse 21, 79104 Freiburg (Germany).
authors.forename: Alexander
authors.initials: A
authors.lastname: Rupp
authors.forename: Nataliya
authors.initials: N
authors.lastname: Roznyatovskaya
authors.forename: Harald
authors.initials: H
authors.lastname: Scherer
authors.forename: Witali
authors.initials: W
authors.lastname: Beichel
authors.forename: Petra
authors.initials: P
authors.lastname: Klose
authors.forename: Carola
authors.initials: C
authors.lastname: Sturm
authors.forename: Anke
authors.initials: A
authors.lastname: Hoffmann
authors.forename: Jens
authors.initials: J
authors.lastname: Tübke
authors.forename: Thorsten
authors.initials: T
authors.lastname: Koslowski
authors.forename: Ingo
authors.initials: I
authors.lastname: Krossing
keywords: Marcus theory; ion pairs; ionic liquids; transport properties; weakly coordinating anions