| Abstract
| - The diffusive properties of an imidazolium room-temperature ionic liquid (RTIL), 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide (DMPImTFSA), are studied from the ionic conductivity and the ion diffusion coefficients measured by pulsed field gradient spin echo NMR. The temperature-dependent 1H, 19F, and 13C NMR spin−lattice relaxation time T1 values were observed, and the 1H T1 for DMPIm showed T1 minima for various protons. According to the Bloemberger−Purcell−Pound (BPP) equation, the correlation time τc values were directly calculated from 1H NMR. By using the 1H τc values, an evaluation of the 13C T1 was attempted for the carbons having protons. The τc estimated for molecular motions of DMPIm changes from 1.3 ns at 253 K to 72 ps at 353 K. The Stokes−Einstein−Debye (SED) model suggests that the τc is too short for the overall molecular reorientation near room temperature. Consequently, the possibility of small-angle molecular rotation is proposed and tentative flip angles are calculated by using the translational diffusion coefficient, the bulk viscosity measured in this study, and the τc obtained from 1H T1 data in the temperature range between 283 and 353 K. The flip amplitude increases with the temperature. DMPIm has isotropic reorientational motions with temperature-dependent amplitude, in addition to fast intramolecular motions such as methylene segmental motions, methyl rotational motion, and conformational exchange of the imidazolium ring. The existence of fast motions of TFSA is also shown. The translational diffusion of the ions is the slowest dynamic process in the present RTIL. Ab initio molecular orbital calculations are performed to understand the geometries of stable complexes of DMPIm+ and TFSA−, and the formation energies from the isolated ions are evaluated. The computed results are important for interpreting the 1H T1 behaviors observed for the imidazolium ring protons.
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