Multinuclear NMR studies on translational and rotational motion for two ionic liquids composed of BF4 anion.
Two ionic liquids (ILs) based on the BF(4)(-) anion are studied by (1)H, (11)B, and (19)F NMR spectroscopy by measuring self-diffusion coefficients (D) and spin-lattice relaxation times (T(1)). The cations are 1-ethyl-3-methylimidazolium (EMIm) and 1-butyl-3-methylimidazolium (BMIm). Since two NMR nuclei ((11)B and (19)F) of BF(4)(-) exhibit narrow lines and high sensitivity, the (11)B and (19)F NMR measurements of D(BF4) and T(1)(BF(4)) were performed in a wide temperature range. The temperature-dependent behaviors of T(1)((19)F) and T(1)((11)B) were remarkably different, although the values of D(BF4)((19)F) and D(BF4)((11)B) almost agreed. Since the Arrhenius plots of T(1)'s for (1)H, (19)F, and (11)B exhibited T(1) minima, the correlation times τ(c)((1)H), τ(c)((19)F), and τ(c)((11)B) were evaluated. The D(cation) and D(BF(4)) were plotted against 1/τ(c)((1)H) and 1/τ(c)((19)F), respectively, and the relationships between translational and rotational motion are discussed. The translational diffusion of the cations is related to molecular librational motion and that of BF(4) is coupled with reorientational motion. The τ(c)((11)B) derived from (11)B T(1) can be attributed to a local jump. From the plots of the classical Stokes-Einstein (SE) equation, the empirical c values, which were originally derived by theoretical boundary conditions, were estimated for each ion. The empirical c(BF(4)) was about 4.4(5), while the c values of the cations were smaller than 4.
National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Center 2, Tsukuba 305-8568, Japan. hayamizu.k@aist.go.jp
Kikuko
K
Hayamizu
Seiji
S
Tsuzuki
Shiro
S
Seki
Yasuhiro
Y
Umebayashi