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NMR evidence of H-bonding in 1-ethyl-3-methylimidazolium-tetrafluoroborate (EMI—BF4) room temperature ionic liquid
Jing-Fang Huang, Po-Yu Chen, I-Wen Sun, S.P. Wang*Department of Chemistry, College of Sciences, National Cheng Kung University
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 In order to end the controversy concerning H-bonding in EMI–BF4 ionic liquid, we have employed NMR techniques to measure diffusion coefficients using proton resonance signals, 13C spin–lattice relaxation times, and nuclear Overhauser enhancements. From the latter two experiments, one can extract the 13C dipole–dipole relaxation rate, [1] which in turn affords the value of correlation time (τc). [2] The 11B quadrupolar relaxation times were also measured to acquire supplementary or comparative information. All these NMR experiments were performed at temperatures ranging from 300 to 360 K with 5 K increments. Besides the proposition of the reliability of PFG-NMR diffusion measurements, it is expected that the NMR spectroscopic techniques employed in this research would lay the foundation for further investigation of inter-ionic interaction and ionic states of ionic liquids. These investigations would in turn provide information for the design or development of new ionic liquids to meet various applications.
A) The ionic states of EMIBM4 ionic liquid
Viewing from EMI: Measurements of Diffusion Coefficients by 1H PFG-NMR
  Scheme. Structure and numbering of atomic positions of 1-ethyl-3- methylimidazolium cation (EMI). Figure 1. The plot of 1/D vs η/T for EMI+ in EMI–BF4.
From the plot of 1/D (inverse of the diffusion coefficient) versus η/T (viscosity over temperature in Kelvin) displayed in Figure 1, two different phases are revealed by the two linear regions. The mean radii of the diffusing spheres in the two phases evaluated from the well known Stokes’ law, [3] are 2.79 Å (300–330 K) and 1.90 Å (335–360 K), respectively. Combined with the reported value of 1.77 Å obtained by reorientation studies,[4] the diffusing sphere at the higher temperatures is ascribed to a free EMI cation. Through the radii of gas-phase BF4 anion obtained by AM1 and ab initio calculations at our labs, in the vicinity of 1.32 Å, it is evident that the EMI cation diffuses in pairs with BF4 anion at lower temperatures.
Viewing from BF4—Measurements of 11B NMR relaxation rates
 Figure 2. The Arrhenius plot of 11B quadrupolar relaxation rate.
The phase change occurred at the temperature 330-335K is confirmed by the temperature dependent 11B quadrupolar relaxation rates, 11B R1q, shown in Figure 2. Since the quadrupolar relaxation is related to the quadrupole coupling constant (χ) by , one may extract the information of the reorientation correlation time, τc.[1] Through the relative slopes, a smaller B-atom containing species under faster rotation (larger slope and hence smaller τc) is observed at higher temperatures. The phase change detected by 11B relaxation, i.e. from the BF4− viewpoint, is informative since it is consistent with the argument that the phase transition arises from decomposition of the ionic pair.
B) Evidence for the H-Bonding between EMI and BF4 ions
The H-bonding is evidenced by the various susceptibilities (of 13C dipolar relaxation of different carbons in EMI) toward the leaving of the BF4 anion as stated below.
 Figure 3. The Arrhenius plot of 13C dipolar relaxation for
(A) C2 (○), NCH3 (●),and NCH2 (□);
(B) CH3 (○), C4 (●), and C5 (□).
The Arrhenius plot of 13C dipole-dipole relaxation rates, R1DD, is presented in Figure 4. These quantities are direct related to the reorientational dynamic behavior [1] of each C—H vector. Leaving of BF4 anion leads to the largest response of the dipolar relaxation of C2. This indicates its reorientation is most hindered by BF4 anion at low temperature range. The nearby NCH2 and NCH3 show smaller but significant susceptibility; whereas the nearly inert response of C(2) and C(5) is seen. Since C2—H bond has highest ionic character and is generally accepted as the H-bonding donor, we concluded that BF4− enters into H-bonding with EMI+. This conclusion is made similar to the H-bonding in BMIBF4 (replacement of ethyl in EMI by butyl) reported by Suarez and co-workers. [5] Furthermore, the relative values of viscosity, as well as the relative values of the correlation time of the C2–H bond, reported for EMI–BF4 and EMI–AlCl4 [6] can now be understood based on the argument of H-bonding in EMI–BF4.
References
- E.D. Becker, High Resolution NMR: Theory and Applications, 2nd ed., Academic Press, New York, 1980 (chap. 9)
- R.T. Boere, R.G. Kidd, in: G.A. Webb (Ed.), Annual Reports on NMR Spectroscopy, Academic Press, New York, 1982
- I.N. Levine, Physical Chemistry, McGraw-Hill, New York, 1978 (chap. 16).
- C.K. Larive, M. Lin, B.S. Kinnear, B.J. Piersma, C.E. Keller, C.E.W.R. Carper, J. Phys. Chem. 102 (1998) 1717.
- P.A.Z. Suarez, S. Einloft, J.E.L. Dullius, R.F. Souz, J.J. Dupont, J. Chim. Phys. 95 (1998) 1626
- A.B. McEwen, H.L. Ngo, K. LeCompte, J.L. Goldman, J. Electrochem. Soc. 146 (1999) 1687;
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