利用電場調變紅外線吸收光譜研究溶劑效應對1,2-二氯乙烷構型平衡的影響

Abstract

1,2-二氯乙烷是一個C-C單鍵內旋轉的簡單樣本模型。1,2-二氯乙烷存在著兩種穩定的構型：間扭異構物以及對扭異構物。間扭異構物是具有極性的，而對扭異構物是沒有極性的。因為這點不同，當1,2-二氯乙烷與溶劑混合的時候，溶劑的極性明顯地影響了兩個異構物之前的平衡關係。換句話說，主導構型平衡的熱力參數，像是自由能變化(□G)以及熵變化(□S)，會隨著1,2-二氯乙烷在溶劑中的微觀環境發生明顯地反應這件事是被期待的。 在本研究中，我們利用電場調變紅外線吸收光譜研究1,2-二氯乙烷以及1,2-二氯乙烷與不同極性的有機溶劑混合(環己烷、四氯化碳、甲苯和氘代氯仿等)對間扭異構物以及對扭異構物的構型平衡的影響。電場調變紅外線吸收光譜是唯一可以實驗測定與構型平衡相關的自由能變化(□G)的方法。自由能變化(□G)的數值由在環己烷中的3.12 (±0.11) kJ mol−1到純1,2-二氯乙烷的1.34 (±0.09) kJ mol−1；此數據與一些極性表示刻度呈現負相關，像是介電常數、偶極矩ET(30) 等。利用已報告的焓變化的數值，我們可以估算熵變化(□S)和兩個異構物的自由體積的比例(Vg/Vt )，提供了其他方法難以取得的1,2-二氯乙烷對溶劑分子之間的分子間作用力的資訊。

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 □G and entropy difference □S, 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 □G associated with the conformational equilibrium. The □G 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 □H, we estimated the entropy difference □S and free-volume ratio Vg/Vt between the two conformers, which provide otherwise unobtainable information on intermolecular interactions between DCE and solvent molecules.