利用電場調變紅外線吸收光譜研究N,N-Dimethyl-p-nitroaniline在混合溶液乙晴和四氯乙烯中溶質的溶解和乙晴的結構

Abstract

本論文主要以電場調變紅外線吸收光譜技術和傅立葉轉換紅外線吸收光譜技術研究N,N-Dimethyl-p-nitroaniline(DMPNA)於乙腈和四氯乙烯混合溶劑中溶質的溶解和乙晴的結構。先前的文獻是利用拉曼光譜技術和電場調變紅外線吸收光譜技術觀測與DMPNA結構類似的p-nitroaniline(PNA)在乙腈與四氯甲烷混合溶劑中的光譜，結果顯示PNA與混合溶劑具有特定的溶質與溶劑的分佈情形。為了瞭解PNA胺基上的氫被雙甲基取代後對溶質與溶劑分佈情形的影響，我們對DMPNA溶液測量乙腈莫耳分率相關性的傅立葉轉換紅外線吸收光譜和角度χ相關性的電場調變紅外線吸收光譜(∆A)，在此，角度χ是指外加電場的與紅外線光源的電場向量方向的夾角。在傅立葉轉換紅外線吸收光譜中，DMPNA的硝基對稱伸縮的波峰隨著乙腈莫耳分率增加而逐漸朝長波長位移，這個結果指出DMPNA並不同於PNA分子會與混合溶劑形成特定的結構。而利用特徵值分解法分析角度χ相關性的紅外線吸收光譜(∆A)也呈現出DMPNA應是以單體的形態存在於溶液中。更有趣的是，經過改善光譜訊號後，於接近DMPNA吸收峰附近能觀察到乙腈吸收度變化的訊號(甲基的變形吸收峰)。我們以溶質不會在溶液中形成特定結構的DMPNA吸收度訊號變化作為光譜內部的標準去估計乙腈的偶極矩，並且再加上文獻中單體DMPNA偶極矩為7.0 D，得到乙腈的偶極矩為5.7 D，此數值約為單體乙腈(3.4 D)分子偶極矩的1.7倍。因此我們認為兩個乙腈分子最可能以頭對尾相接的線型雙體結構存在於此混合溶液中。

N,N-Dimethyl-p-nitroaniline (DMPNA) in mixed solvents of acetonitrile (ACN) and tetrachloroethylene (C2Cl4) was studied with FT-IR spectroscopy and IR electroabsorption spectroscopy. Previous studies using Raman and IR electroabsorption spectroscopy show that p-nitroaniline (PNA), an analogue of DMPNA, forms specific solvation structures with ACN in ACN/CCl4. To examine the effect of N,N-dimethyl substitution on the formation of those solvation structures, we measured the ACN mole fraction dependence of FT-IR spectra and the angle χ dependence of IR electroabsorption (∆A) spectra of DMPNA in ACN/C2Cl4. Here χ is the angle between an applied electric field and the electric field vector of IR probe light. In the FT-IR spectra, the NO2 symmetric stretch band of DMPNA shows continuous redshift with increasing ACN mole fraction, indicating that, unlike PNA, DMPNA does not exhibit specific solvated forms in the mixed solvents. Singular value decomposition analysis of the χ-dependent ∆A spectra also supports this conclusion. More interestingly, through efforts to improve the data, we observed a ∆A signal (CH3 symmetric deformation of ACN) in the vicinity of the DMPNA band. We used the ∆A signal of DMPNA, which has been shown to form no specific solvation structures, as an internal intensity standard to evaluate the dipole moment of ACN. Using the value of the dipole moment μP, of DMPNA in the literature, we obtained μP of ACN as 5.7 D, which is about 1.7 times as large as that of individual ACN molecules (~3.4 D). Thus a head-to-tail linear dimer in which two ACN molecules align linearly pointing to the same direction is most plausible as the association structure in the mixed solvents.