研究水溶解於1,4-二氧陸圜的電場變調紅外線吸收光譜學

Abstract

無論是靜態或動態的水都是一種複雜的液體，在本研究中我們用全新的觀點來討論靜態水分子對外加電場的反應。我們用電場變調紅外線吸收光譜來研究水溶解於1,4-二氧陸圜的O–H伸縮振動範圍，我們的技術可以以高靈敏度偵測到外加電場對紅外線吸收光譜的變化，提供分子定量特性的分析，如永久偶極矩和可極化率等，而這些物性可以直接反應出分子的溶解結構和環境。 首先我們用多變量曲線分辨率方法分析水溶解於1,4-二氧陸圜的濃度變化傅立葉轉換紅外線吸收光譜(濃度為0-2M)，所觀察到的濃度依賴性光譜可經由假設兩個獨立的成分組合得到，我們命為單一水分子被1,4-二氧陸圜包圍和自組的小群集水分子。接著我們偵測水溶解於1,4-二氧陸圜的電場的紅外線吸收光譜(□A)，可成功測到□A的訊號大小約為10−6。我們也測得一系列改變角度□的□A光譜，並用奇異值分析方法來分析，可成功分離出□相關和□不相關成分光譜，擬合光譜後的結果和目前為止對於其他液體分子的研究相比，本樣品對於電場回應的主要是由電子極化的貢獻(即偶極矩的變化和/或極化度的振動激發)，而非方向的取向異性貢獻。

Water is an exceedingly complicated liquid in both static and dynamic aspects. In the present study, we aimed to shed new light on the static molecular properties of water through investigating its response to an external electric field modulation. We studied the O–H stretch region of water dissolved in 1,4-dioxane using infrared (IR) electroabsorption spectroscopy. This technique is capable of detecting with high sensitivity the changes in IR absorption spectrum induced by an externally applied electric field. It provides quantitative information on molecular properties such as the permanent dipole moment and the polarizability, which sharply reflect the structure and local environments of the molecule. First, using a multivariate curve resolution approach, we analyzed a series of FT-IR spectra of water dissolved in 1,4-dioxane at different concentrations of water ranging from 0 to 2 M. The results show that the observed concentration-dependent spectra can be reproduced well by assuming two independent components. We assigned these components as isolated water species surrounded by 1,4-dioxane and those in small water ensemble (cluster). Next, we measured IR electroabsorption (□A) spectra of the O–H stretch band of water dissolved in 1,4-dioxane. □A signal of the order of 10−6 was successfully detected. We recorded the □A spectrum by changing the angle, □□ between the direction of the applied field and the polarization vector of the IR light. The □-dependence of the □A spectra was analyzed with a singular value decomposition approach, yielding the □-dependent and □-independent component spectra. Fitting analysis of both spectra suggests that, in contrast to other molecular liquids studied so far, water dissolved in 1,4-dioxane seems to respond to the electric field via the electronic polarization (i.e., changes in dipole moment and/or polarizability upon vibrational excitation) rather than orientational anisotropy.