| Summary: | 碩士 === 國立交通大學 === 應用化學系碩博士班 === 100 === 1,2-Dichloroethane (DCE) is a simple model system for internal rotation around a C–C single bond. DCE occurs as two stable conformers: gauche and trans conformers. The gauche conformer is polar, whereas the trans conformer is nonpolar. Because of this difference, the equilibrium between the two conformers is profoundly affected by solvent polarity when DCE is mixed with solvent. In other words, the thermodynamic parameters dictating the conformational equilibrium, such as the free energy difference ?孱 and entropy difference ?巽, are anticipated to sharply reflect microscopic environments around DCE in solvent.
In the present study, we use infrared (IR) electroabsorption spectroscopy to study the trans/gauche conformational equilibrium of pure DCE and DCE mixed with organic solvents having different polarity (cyclohexane, CCl4, toluene, and d-chloroform). IR electroabsorption spectroscopy is the only method that can experimentally determine ?孱 associated with the conformational equilibrium. The ?孱 value changes from 3.12 (±0.11) kJ mol−1 in cyclohexane to 1.34 (±0.09) kJ mol−1 in pure DCE; it negatively correlates with polarity scales such as dielectric constant, dipole moment, and ET(30) value. Using the reported values of the enthalpy difference ?寒, we estimated the entropy difference ?巽 and free-volume ratio Vg/Vt between the two conformers, which provide otherwise unobtainable information on intermolecular interactions between DCE and solvent molecules.
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