核鹼基模型化合物4(3H)-嘧啶酮於溶液相的電場調變 紅外線吸收光譜學

Abstract

本論文中，作者利用電場調變紅外線吸收光譜學研究核鹼基模型化合物，4(3H)-嘧 啶酮於溶液態對外加電場的反應。先前的文獻指出此化合物於溶液態將同時存在單體以 及雙體，然而目前仍沒有實驗方法直接研究此化合物及其雙體的化學平衡。電場調變紅 外線光譜學可靈敏地偵測分子對外加電場的反應。且已被證實為一種可研究在溶液相同 時存在多物種系統的工具。本實驗為了同時偵測單體及雙體，主要研究碳氧雙鍵振動的 波數區間且成功地偵測到約百萬分之一的吸收值變化。為了獲得定量的分子資訊， 我 們量測一系列的角度c相關實驗， c為外加電場以及紅外線的電場極化向量的夾角。實 驗結果與先前研究過的極性分子系統相當不同，此分子並無出現明顯的角度相關性。 為 了更仔細的檢視角度相關實驗的結果，我們利用特徵值分解分析(SVD)成功將光譜分解 為角度相關以及角度無關兩個成分。於角度無關成分中並沒有發現到原本預期的單雙體 平衡訊號。原因可能是電場對極性分子(單體)的穩定能並不足夠使以分子間氫鍵結合的 雙體(若為平面則無偶極矩)分離。角度相關成分對於原光譜的貢獻度比起先前研究過的 極性分子不尋常的小，我們認為其原因是振動躍遷偶極與偶極矩的夾角相當接近會讓轉 動極化訊號(於先前發表的文獻中通常是最主要的貢獻)消失的54.7°(魔術角度)。據我們 所知，本研究應是第一個研究核鹼基模型化合物4(3H)-嘧啶酮在室溫溶液態對外加電場 反應的實驗。

In this thesis, 4(3H)-Pyrimidinone, an important model compound of pyrimidine base, in p-dioxane solution was studied with infrared (IR) electroabsoprtion spectroscopy. A previous study using FT-IR spectroscopy showed that the monomer and dimer of 4(3H)-pyrimidinone, coexist in the solution phase. Although it is crucial to understand the effects of an electric field on this molecule with respect to molecular structure and monomer–dimer equilibrium, direct experimental studies are yet to be done. IR electroabsorption spectroscopy can detect the molecular responses to an externally applied electric field with high sensitivity. In particular, it has been proven a powerful technique for studying solution sample in which different molecular species coexist. We measured IR electroabsorption (DA) spectrum in the C=O stretching region of 4(3H)-pyrimidinone in p-dioxane in which both monomer and dimer appear. In a series of c-dependent DA spectra, we did not observe marked c dependence of the DA signals of both monomer and dimer. Here, angle c is the angle between the direction of the applied field and the electric field vector of the IR light. To examine the c-dependence of the DA spectra quantitatively, we performed singular value decomposition (SVD) analysis, in which the whole set of the spectra were decomposed into the c-dependent and c-independent spectral components. The c-independent component, shows the prominent electronic polarization signal in contrast to other molecular liquids studied before. In addition, the DA signal arising from equilibrium change between the monomer and dimer was not observed in the c-independent component, which implies that the dipolar stabilization with the applied electric field (~10MVm-1) may not be strong enough to dissociate the dimer. The absence of a large c dependent component is rather unusual for a polar molecule having a permanent dipole moment. We attribute this observation to the angle between the vibration transition moment and the permanent dipole moment which is very close to the magic angle 54.7°, at which the orientational polarization signal vanishes. Results of our least-squares fitting iii analysis of the c-dependent spectral component are consistent with our interpretation. The present IR electroaborption study of 4(3H)-pyrimidinone has revealed experimentally for the first time the electric-field effects of a nucleobase model in a biologically relevant environment (i.e., ambient solution).