以IR研究3,5-二甲基-1H-吡咯-2-甲酸乙酯構型轉換、氫鍵自結合現象及3,5-二甲基-2-乙醯基吡咯氫鍵自結合現象

Abstract

利用IR光譜研究3,5-二甲基-2-乙醯基吡咯和3,5-二甲基-1H-吡咯-2-甲酸乙酯在溶液中的自結合。3,5-二甲基-2-乙醯基吡咯在稀薄溶液下呈現順式單體、環狀雙體兩者共存。處理單體N-H基吸收度數據公式 (2-13) 及雙體N-H基吸收度數據公式 (2-17) ，可得到單、雙體N-H基伸縮振動總莫耳吸收係數εm 、εd及自結合平衡常數K。3,5-二甲基-1H-吡咯-2-甲酸乙酯在稀薄溶液下呈現順式、反式單體及雙體三者共存。本實驗室推導新的公式，企圖從線性公式 (2-33) 的斜率PC、截距QC、式 (2-36) 的斜率Pt、截距Qt及式 (2-39) 的斜率Pd、截距Qd中求取順式、反式單體及雙體N-H基的總莫耳吸收係數εC、εt、εd及單體構型轉換平衡常數K1和自結合平衡常數K2。但是除了雙體總莫耳吸收係數εd可由Qd得到外，其餘並不可得。此由於PC、Pt、QC、Qt相依，可由Jacobian函數檢視證明之。為了克服相依性所帶來這個困境，以PBE1PBE/6-311++G**計算出εC/εt，並與實驗求得的反式、順式單體N-H基吸收帶的吸收度At、AC代入K1= (At/AC)(εC/εt)中。K1再代入式PC、Pd、QC及Qt中，即可求出εC、εt、K2。 當溶劑的介電常數越大，單、雙體N-H基的伸縮振動頻率會往低頻位移，此與KBM預測的趨勢相符。隨溶劑介電常數的增加，3,5-二甲基-1H-吡咯-2-甲酸乙酯單體構型轉換標準焓ΔH1O、自結合標準焓ΔH2O皆減少。當溶劑的介電常數增加使得單、雙體N-H基吸收帶的半高寬變寬。實驗的結果發現，雙體N-H基總莫耳吸收係數大於單體N-H基總莫耳吸收係數。G. C. Pimentel與T. Ebata認為這與質子受體、質子予體之間電荷分佈急劇改變有關，進而使偶極距改變率dμ/dr增加所致。本實驗在探討溶劑對單、雙體N-H基伸縮振動強度的影響時套用了M. I. Nasser和E. Hirota的方法。但溶劑對強度的影響不是很顯著，因此無法用此方法解釋，但定性來說隨溶劑極性越大，單、雙體N-H基強度越強。

The self-association of 2-Acetyl-3,5-dimethylpyrrole (ADP) and Ethyl 3,5-dimethyl-1H-pyrrole-2-carboxylate (EDPC) in various solvents have been studied using infra-red spectra. The molar absorption coefficient of N-H stretching for ADP monomer and ADP dimer are obtained from the equation derived from the concentration dependent integrated absorbance of monomer bands and those of dimer bands, respectively. So does the dimerization constant obtained independently from them. The letter offers an opportunity to check the consistency of the determination. As for EDPC, the spectra is found that two N-H bands exist in monomer region. This system is a monomer-dimer self-association accompanying a cis-trans inter-conversion in monomer. Our three linear plots respectively for cis, trans monomer and dimer bands are unable to give the molar absorption coefficients of cis and trans monomer N-H stretching εC, εt and cis-trans interconversion constant K1, dimerization constant K2, except for the molar absorption coefficient of dimer band. To overcome this difficulty, we resort to quantum chemical calculation using PBE1PBE/6-311++G** functional to give the ratioεC/εt. The letter enables us to estimate K1 from the ratio of integrated absorbance At/AC. With K1 known, other parameters K2, εC andεt can be solved. The solvent effect on the frequency shift, band width, band intensity and enthalpy of dimerization are also discussed.