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Pure Appl. Chem., 2004, Vol. 76, No. 1, pp. 255-261

Molecular structure, reorientational dynamics, and intermolecular interactions in the neat ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate

J. H. Antony, D. Mertens, Tobias Breitenstein, Andreas Dölle, P. Wasserscheid and W. R. Carper

Institut für Physikalische Chemie,Rheinisch-Westfälische Technische Hochschule, Aachen, 52056 Aachen, Germany; Institut für Technische und Makromolekulare Chemie, Rheinisch-Westfälische Technische Hochschule Aachen, 52056 Aachen, Germany; Department of Chemistry, Wichita State University, Wichita, KS 67260-0051, USA

Abstract: Results on the molecular and liquid structure and the reorientational dynamics are reported for the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). In quantum-chemical calculations for [BMIM][PF6] in the gas phase, hydrogen bonding between the proton at carbon 2 in the aromatic ring and the fluorine atoms of the hexafluorophosphate anion was found. From the analysis of 13C relaxation data, the reorientational motions were evaluated, and the Vogel-Fulcher-Tammann and Arrhenius activation energies for the overall and internal reorientational motions, respectively, of the different 13C-1H vectors are given as well as correlation times at 300 K. By performing molecular dynamics (MD) simulations, pair distribution functions between moieties in the cation and the phosphorous atom in the anion were determined. The pair distribution function for the proton at carbon 2 exhibits a particular sharp and strong maximum indicating a strong interaction with the anion. The quantum-chemical calculations, the motional parameters, and the results from the MD simulations support the existence of hydrogen bonding and the formation of ion pairs in the ionic liquid.