title: Towards a molecular understanding of cation-anion interactions--probing the electronic structure of imidazolium ionic liquids by NMR spectroscopy, X-ray photoelectron spectroscopy and theoretical calculations.
abstract: Ten [C(8)C(1)Im](+) (1-methyl-3-octylimidazolium)-based ionic liquids with anions Cl(-), Br(-), I(-), [NO(3)](-), [BF(4)](-), [TfO](-), [PF(6)](-), [Tf(2)N](-), [Pf(2)N](-), and [FAP](-) (TfO=trifluoromethylsulfonate, Tf(2)N=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 [Tf(2)N](-) were investigated by using X-ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While (1)H 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 XPS 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.
authors.affiliation: Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany.
authors.forename: Till
authors.initials: T
authors.lastname: Cremer
authors.forename: Claudia
authors.initials: C
authors.lastname: Kolbeck
authors.forename: Kevin R J
authors.initials: KR
authors.lastname: Lovelock
authors.forename: Natalia
authors.initials: N
authors.lastname: Paape
authors.forename: René
authors.initials: R
authors.lastname: Wölfel
authors.forename: Peter S
authors.initials: PS
authors.lastname: Schulz
authors.forename: Peter
authors.initials: P
authors.lastname: Wasserscheid
authors.forename: Henry
authors.initials: H
authors.lastname: Weber
authors.forename: Jens
authors.initials: J
authors.lastname: Thar
authors.forename: Barbara
authors.initials: B
authors.lastname: Kirchner
authors.forename: Florian
authors.initials: F
authors.lastname: Maier
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Steinrück