Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 16, Issue 30, Pages 9018-9033Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201001032
Keywords
charge transfer; density functional calculations; electronic interactions; ionic liquids; NMR spectroscopy; X-ray photoelectron spectroscopy
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Funding
- German Science Foundation (DFG)
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Ten [C(8)C(1)Im](+) (1-methyl-3-octylimidazolium)-based ionic liquids with anions Cl-, Br-, I-, [NO3](-), [BE4](-), [TfO](-), [PF6](-), [Tf2N](-), [Pf(2)N](-), and [FAP](-) (TfO=trifluoromethylsulfonate, Tf2N=bis(trifluoromethylsulfonyl)imide, Pf(2)N=bis(pentafluoroethylsulfonyl)imide, FAP=tris-(pentafluoroethyl)trifluorophosphate) and two [C(8)C(1)C(1)Im](+) (1,2-dimethyl-3-octylimidazolium)-based ionic liquids with anions Br- and [Tf2N](-) were investigated by using X-ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While H-1 NMR spectroscopy is found to probe very specifically the strongest hydrogen-bond interaction between the hydrogen attached to the C-2 position and the anion, a comparative X.PS study provides first direct experimental evidence for cation-anion charge-transfer phenomena in ionic liquids as a function of the ionic liquid's anion. These charge-transfer effects are found to be surprisingly similar for [C(8)C(1)Im](+) and [C(8)C(1)C(1)Im](+) salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.
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