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 |
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Authors: | Till Cremer Claudia Kolbeck Kevin?R?J Lovelock Dr Natalia Paape René Wölfel Peter?S Schulz Dr Peter Wasserscheid Prof?Dr Henry Weber Jens Thar Barbara Kirchner Prof?Dr Florian Maier Dr Hans‐Peter Steinrück Prof?Dr |
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Institution: | 1. Lehrstuhl für Physikalische Chemie II, Universit?t Erlangen‐Nürnberg, Egerlandstrasse 3, 91058 Erlangen (Germany), Fax: (+49)?9131‐85‐28867;2. School of Chemistry, The University of Nottingham, Nottingham, NG7 2RD (UK);3. Lehrstuhl für Chemische Reaktionstechnik, Universit?t Erlangen‐Nürnberg, Egerlandstrasse 3, 91058 Erlangen (Germany);4. Wilhelm‐Ostwald‐Institut für Physikalische und Theoretische Chemie, Linnéstra?e 2, 04103 Leipzig (Germany) |
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Abstract: | Ten C8C1Im]+ (1‐methyl‐3‐octylimidazolium)‐based ionic liquids with anions Cl?, Br?, I?, NO3]?, BF4]?, TfO]?, PF6]?, Tf2N]?, Pf2N]?, and FAP]? (TfO=trifluoromethylsulfonate, Tf2N=bis(trifluoromethylsulfonyl)imide, Pf2N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two C8C1C1Im]+ (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 1H NMR spectroscopy is found to probe very specifically the strongest hydrogen‐bond interaction between the hydrogen attached to the C2 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 C8C1Im]+ and C8C1C1Im]+ 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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Keywords: | charge transfer density functional calculations electronic interactions ionic liquids NMR spectroscopy X‐ray photoelectron spectroscopy |
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