| Summary: | 碩士 === 國立交通大學 === 應用化學系碩博士班 === 99 === 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.
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