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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
Author(s) -
Cremer Till,
Kolbeck Claudia,
Lovelock Kevin R. J.,
Paape Natalia,
Wölfel René,
Schulz Peter S.,
Wasserscheid Peter,
Weber Henry,
Thar Jens,
Kirchner Barbara,
Maier Florian,
Steinrück HansPeter
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201001032
Subject(s) - ionic liquid , chemistry , x ray photoelectron spectroscopy , hydrogen bond , ion , ionic bonding , imide , spectroscopy , nuclear magnetic resonance spectroscopy , inorganic chemistry , molecule , stereochemistry , organic chemistry , nuclear magnetic resonance , catalysis , physics , quantum mechanics
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.