Molecular Reorientational Dynamics of the Neat Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate by Measurement of 13C Nuclear Magnetic Relaxation Data

• Published in: Chemphyschem Vol. 4; no. 6; pp. 588 - 594
• Main Authors: Antony, Jürgen H., Mertens, Dirk, Dölle, Andreas, Wasserscheid, Peter, Carper, William R.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 16.06.2003; WILEY‐VCH Verlag; Wiley
• Subjects: Atomic and molecular physics; Correlation times in molecular dynamics; Exact sciences and technology; green chemistry; hydrogen bonds; ionic liquids; molecular dynamics; Molecular properties and interactions with photons; NMR spectroscopy; Physics; Properties of molecules and molecular ions; Phosphates; Imidazole derivatives; Ionic compound; Nitrogen heterocycle; Molecular dynamics; Liquid phase; Experimental study; green chemistry; Relaxation time; Reorientation diffusion; Hydrogen bonds; Correlation time; Molecular reorientation; NMR spectra; ionic liquids; NMR spectroscopy; Carbon 13; Organic compounds
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200200603

The reorientational dynamics of the ionic liquid 1butyl‐3‐methylimidazolium hexafluorophosphate ([BMIM]PF6) were studied over a wide range of temperatures by measurement of 13C spin–lattice relaxation rates and NOE factors. The reorientational dynamics were evaluated by performing fits to the experimental relaxation data. Thus, the overall reorientational motion was described by a Cole–Davidson spectral density with a Vogel–Fulcher–Tammann temperature dependence of the correlation times. The reorientational motion of the butyl chain was modelled by a combination of the latter model for the overall motion with a Bloembergen–Purcell–Pound spectral density and an Arrhenius temperature dependence for the internal motion. Except for C2 in the aromatic ring, an additional reduction of the spectral density by the Lipari–Szabo model had to be employed. This reduction is a consequence of fast molecular motions before the rotational diffusion process becomes effective. The C2 atom did not exhibit this reduction, because the librational motion of the corresponding C2H vector is severely hindered due to hydrogen bonding with the hexafluorophosphate anion. The observed dynamic features of the [BMIM]+ cation confirm quantum‐chemical structures obtained in a former study. Hydrogen bonding plays a key role in the ionic liquid [BMIM]PF6, of which the calculated structure of one ion pair is shown in the picture. It occurs mainly between C2H of the heterocyclic ring and the PF6− anion. This specific interaction strongly influences the molecular structure and dynamics in the liquid and may explain its relatively high viscosity and some of its other specific properties. In particular, the effect of the hydrogen bonds on the reorientational dynamics of [BMIM]PF6 was investigated.