| Summary: | 碩士 === 國立成功大學 === 化學系碩博士班 === 95 === The high ionicity of ionic liquids in propylene carbonate (PC) has been found, through 19F NMR chemical shift, arising from the acidic character of the propylene framework in the basic PC organic molecule. We have applied this “base-acid dual property argument to increase the molar conductivity in BMIPF6/CH3CH2OH solutions by fluorinations of the methyl hydrogens. The fluorinated ethanol, CF3CH2OH, has been thus selected as the co-solvent in use with BMIPF6 ionic liquid.
Through analysis of the diffusion coefficient ratios acquired by 1H resonance of CF3CH2OH to that of BMI cation, one finds pleasantly improved decomposition of the clusters in CH3CH2OH. The basic character of CF3CH2OH has been identified by 19F and 1H chemical shift variations. It is interesting that (1)hyperconjugation of the solvent’s CF3-donor group prefers H4/H5 of BMI cation to H2, which can be explained by the steric hindrance of PF6 anion; (2) the various-temperature 19F spin-lattice relaxation times (T1) of CF3 show phase change, e.g. (BMIPF6)( BMIPF6)PF6��( BMIPF6)PF6 +BMI+ +PF6 followed by e.g. (BMIPF6)PF6�袈F6 +BMI +PF6, in the temperature 310-320K. This phase change can be rationalized by the slippery of the BMI cation from (BMIPF6)nPF6 hyper anions, n is more likely 1 or 2 in this research.
More significantly the diffusion coefficient ratios obtained by 19F signals, DD(F), reveal a larger value than those acquired by 1H signals, DD(H), for the ��=0.25 solution at various temperature. The larger DD-value of former experiments reveal that dissociation of PF6 anion is less than that of BMI cation. This result strongly suggests that hyper anions as stated above is more abundant than the hyper cations. It is very informative that DD(F) turns smaller than DD(H) especially at high concentration of CF3CH2OH. This reveals that the above-sated phase change occur at room temperature for the ��=0.80 solution. Vanishing of hyper anions allows the DD-value more close to the value of ion radius (rBMI>rPF6) to solvent radius.
The hyper anion argument can (1)supply an alternative explanation of the smaller diffusion coefficient measured for PF6 despite it’s radius is smaller than the cation; (2)correlate the molar conductivity data with diffusion coefficient data acquired for BMIPF6 in CF3CH2OH, as well as those acquired for BMIPF6 in CH3CH2OH.
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