藉由紅外光譜分別探討伴隨/不伴隨構型轉換現象的2-吡咯甲酸甲酯/2-(三氟乙醯基)吡咯等兩種化合物的自結合現象

Abstract

紅外光譜被用以探討2-(三氟乙醯基)吡咯與2-吡咯甲酸甲酯分子於溶劑中的自結合現象及溶劑效應。2-(三氟乙醯基)吡咯單體利用N-H…C=O氫鍵構成環狀雙體。自結合平衡常數K與單、雙體的總莫耳吸收係數可藉由單體與雙體N-H伸縮振動吸收帶的總吸收度計算而得。2-吡咯甲酸甲酯有兩種能量相近的順、反式單體共存於溶液中，而雙體主要由能量較低的順式單體以環狀的方式構成。我們利用順、反式單體與雙體N-H伸縮振動吸收帶的總吸收度，推導出三條處理此種系統的線性方程式。然而，唯有雙體的總莫耳吸收係數εd可獨立求得。其他參數如自結合平衡常數K2，順、反式單體轉換平衡常數K1，以及順、反式單體的總莫耳吸收係數(分別為εcis及εtrans)等參數，包含於用以表示斜率及截距的函數中。我們已由Jacobian定理證明，包含K1、K2、εcis及εtrans等參數的斜率與截距函數，彼此間為函數相依，導致上述的參數無法獨立求得。故我們利用PBE1PBE/6-311++G**計算的順、反式單體總莫耳吸收係數比，藉此克服函數相依所帶來的困境，並計算出所需的參數。 溶劑效應表現在2-吡咯甲酸甲酯的順、反式單體轉換反應焓ΔH1，以及2-吡咯甲酸甲酯或2-(三氟乙醯基) 吡咯的自結合反應焓ΔH2。主要來自於溶質偶極矩與反應場作用的強弱，ΔH1及|ΔH2|皆會隨溶劑介電常數的增加而減少。N-H基伸縮振動頻率的溶劑效應與Buckingham所提出的關係式具有相同的趨勢。溶劑效應造成吸收峰半高寬的變寬現象，可藉由Fermi’s Golden rule與反應場理論得到良好的解釋。然而，Polo-Wilson方程式對吸收強度的溶劑效應，在定量上與實驗不完全相符，但於定性上有隨溶劑介電常數增加而變大的趨勢。此外，我們亦由實驗數據提出檢驗系統中線性雙體存在與否的關係式，並探討自結合如何造成溶液的非理想性。由PBE1PBE/6-311++G**預測MPC的N-H基伸縮振動頻率與我們的實驗結果在定性及定量上皆相當一致。吸收強度的預測則與實驗結果在定性上有相同的趨勢。

Infrared spectroscopy has been employed to study the molecular self-association of the methyl pyrrole-2-carboxylate (MPC) and 2-(trifluoroacetyl)pyrrole(TFP) in various solvents at different temperatures. In the dilute solution, the IR spectra of TFP shows two bands in the N-H stretching vibrational regions. The band in higher vibrational frequency is assigned to cis-monomer, and the other one is assigned to the cyclic dimer. The dimerization constant, K, can be obtained independently either from the monomer or dimer bands by means of the two linear equations (2-39) or (2-40), respectively. Furthermore, the experimental accuracy can be examined by comparing the dimerization constants determined from equation (2-39) and (2-40). The IR spectra of MPC shows three prominent N-H stretching vibrational bands in the dilute solution. These three bands are assigned to trans-monomer, cis-monomer and cyclic dimer from the highest vibrational frequency to the lower one. In order to determine the cis-trans interconversion equilibrium constant(K1), the dimerization constant(K2), the molar absorption coefficients of N-H stretching vibrational mode from trans monomer, cis monomer and dimer (represented by εt, εc and εd, respectively), three linear equations (2-22), (2-21) and (2-23) have been derived corresponding to above the three absorption bands. However, only the εd can be determined independently from the intercept function Qd of linear equation (2-23). The intercepts of linear equation (2-21) and (2-22), and the slopes of linear equation (2-21),(2-22) and (2-23) are functional dependently which has been proved by the nullity of the Jacobian. So the parameters included in the above slopes and intercepts, such as K1, K2, εt and εc are impossibly determined. In order to overcome the difficulties brought from the functional dependence, we resort to the reliable intensity ratio of cis and trans monomers calculated by the PBE1PBE/6-311++G** level. Solvent effects on the enthalpies of cis-trans monomer interconversion(ΔH1) and of self-association(ΔH2) are maily from the interaction between dipole moment of solutes and reation field caused by solvents. Both ΔH1 and ΔH2 decrease with the increase of dielectric constant. N-H stretching vibrational frequency decreases with the enhancement of solvent polarity. This trend is consistent with Buckingham’s relation. Solvent effects on the band-widths of N-H stretching vibrational bands, Δṽ1/2, can be explained by Fermi’s golden rule with the conjunction of reaction field theory. Nevertheless, the relative intensities in various solvents comparing with the prediction from Polo-Wilson equation is quantitatively poor, but increase qualitatively with the Polo-Wilson theory. Besides, we proposed a plot, Ad/Am2 against [B]o, where Ad and Am are the dimer and monomer bands of N-H stretching vibrational modes, respectively, and [B]o is the prepared concentration, to examine the existence of linear dimers in solution. We discussed that how does the molecular self-association cause the non-ideal behavior in non-aqueous solutions. The N-H stretching vibrational frequencies of MPC predicted by PBE1PBE/6-311++G** level are quantitatively and qualitatively consistent with the experimental IR spectra, but the intensities are only qualitatively constsitent with the experimental results.