Molecular structure, reorientational dynamics, and intermolecular
interactions in the neat ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate
J. H. Antony, D. Mertens, T. Breitenstein, A. D�lle, 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.
*Lecture presented at the European Molecular Liquids
Group (EMLG) Annual Meeting on the Physical Chemistry of Liquids: Novel
Approaches to the Structure, Dynamics of Liquids: Experiments, Theories,
and Simulation,Rhodes, Greece, 7-15 September 2002. Other presentations
are published in this issue, pp. 1-261.