| Summary: | 碩士 === 國立成功大學 === 化學系碩博士班 === 97 === Nuclear magnetic resonance techniques have been applied to investigate the influences of diluent type, diluent content and temperature on the chemical shift, diffusion coefficient and the relaxation time for
1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (HEMIMBF4).The macroscopic properties, like viscosity, density and conductivity of the mixture were measured.
To understand further the details of ionic diffusion and conductivity, molecular dynamics simulations were also conducted.
It’s known from the experiment that the 1H chemical shift difference has the order：H10＞H2＞H5∼H4, which indicates that the hydroethyl hydrogen atom of HEMIM+ ion forms a stronger hydrogen bond with BF4－ ion and diluent molecules than any other hydrogen atoms. The result is in accord with the partial charge obstained from the simulations.
The NOESY spectra shows that most of the ions in pure HEMIMBF4 aggregate to form large clusters, the size of which diminishes with the increase of diluent content, which is further confirmed by the simulations.
In general, the diffusion coefficient of BF4－ ion is greater than that of HEMIM+ ion. The diffusion rate of HEMIM+ at a acetonitrile (AN) molar fraction (XAN) of 0.8 is far greater than that at 0.7, owing to the rapid increase of coordinating acetonitrile molecules around HEMIM+ ion.
As the diluent content increases, the viscosity of solution decreases, but the relaxation time, diffusion rate and specific conductivity increases. These phenomena can be interpreted with the coordination number, free ion fraction and ionic cluster size that are obtainable from the simulations.
At constant temperature and diluent concentration, benzonitrile molecule has stronger interaction with HEMIM+ than AN. For HEMIMBF4 / BN system, the Stokes-Einstein radius is smaller than that of AN containing system, which leads to the high diffusion rate and conductivity.
The origin can be traced back to the differences in the coordination number, free ion fraction, diluent size and solution viscosity.
Based on this work, the surroundings around the HEMIM+ ion is now clear, which is believed to be helpful for the microscopic explanation of experimental observations.
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