Nuclear magnetic resonance study of the dynamics of imidazolium ionic liquids with -CH2Si(CH3)3 vs -CH2C(CH3)3 substituents.

Trimethylsilylmethyl (TMSiM)-substituted imidazolium bis(trifluoromethylsulfonyl)imide (NTf(2)-), and tetrafluoroborate (BF(4)-) ionic liquids (ILs) have lower room-temperature viscosities by factors of 1.6 and 7.4, respectively, than isostructural neopentylimidazolium ILs. In an attempt to account for the effects of silicon substitution in imidazolium RTILs and to investigate the ion dynamics, we report nuclear magnetic resonance (NMR) measurements of 1H (I = 1/2) and 19F (I = 1/2) spin-lattice relaxation times (T1) and self-diffusion coefficients (D) as a function of temperature for ILs containing the TMSiM group and, for comparison, the analogous neopentyl group. The 1H and 19F nuclei probe the dynamics of the cations and anions, respectively. The low-temperature line shapes were determined to be Gaussian, and the onset of the rigid lattice line width is correlated with the measured glass transition temperature. The spin-lattice relaxation data feature a broad T1 minimum as a function of inverse temperature for both nuclear species. The Arrhenius plots of the diffusion data for both nuclear species are found to exhibit Vogel-Tammann-Fulcher curvature. Analysis of the eta and D data generally show fractional Stokes-Einstein behavior D proportional to (T/eta)m. This is most prominent in the neopentylimidazolium BF(4)- IL with m approximately 0.66.