Fourier Transform Near-Infrared Spectra of N-Methylacetamide: Dissociation and Thermodynamic Properties in Pure Liquid Form and in CCl4 Solutions
Fourier Transform Near-Infrared Spectra of N-Methylacetamide: Dissociation and Thermodynamic Properties in Pure Liquid Form and in CCl4 Solutions
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DOI：
10.1366/0003702944029460
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发表时间：
1994-09
影响因子：
3.5
通讯作者：
Yongliang Liu;M. A. Czarnecki;Y. Ozaki
中科院分区：
文献类型：
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作者：
Yongliang Liu;M. A. Czarnecki;Y. Ozaki
在303-368K和283-343K温度范围内，测量了纯液态N-甲基乙酰胺(NMA)和四氯化碳(CCl4)溶液中的FT-NIR光谱。对于纯液体，FT-NIR光谱在NH伸缩模式的第一泛音区域给出了一个自由NH基团、至少5个自由端NH基团和至少6个氢键NH基团的谱带，显示了FT-NIR光谱研究NMA氢键的潜力。NMA在纯液体中的光谱随温度的变化表明，随着温度的升高，NMA的长链齐聚物分解为单体和二聚物种。对于CCl4溶液，用单体NMA的NH伸缩模式的第一泛音谱带的强度研究了离解程度和离解过程的热力学性质。在4×10−3M以下，NMA完全以单体形式存在，并从吸光度-浓度图中得到NH伸缩模式第一泛音的摩尔吸收系数。计算了不同浓度溶液的解离度和解离平衡常数K，并用它们计算了氢键甲基丙烯酸甲酯解离为单体物种的热力学参数(ΔH、ΔS和ΔG)。结果表明，随着浓度的降低，熵(ΔS)对标准自由能(ΔG)的贡献变大。在稀溶液中，似乎很可能主要存在短链低聚物，它们很容易解离成单体物种。
FT-NIR spectra have been measured for N-methylacetamide (NMA) in pure liquid form and in carbon tetrachloride (CCl4) solutions over temperature ranges of 303–368 K and 283–343 K, respectively. For the pure liquid, the FT-NIR spectra give one band due to a free NH group, at least five bands due to free end NH groups, and at least six bands due to hydrogen-bonded NH groups in the region of the first overtone of the NH stretching mode, demonstrating the potential of FT-NIR spectroscopy for investigating the hydrogen-bonding of NMA. The temperature-dependent spectral changes of NMA in the pure liquid indicate that, with increasing temperature, longer-chain oligomers of NMA dissociate into monomeric and dimeric species. For the CCl4 solutions, the intensity of a band assigned to the first overtone of the NH stretching mode of the monomeric NMA has been employed to study the degree of dissociation and thermodynamic properties for the dissociation process. Below 4 × 10−3 M, NMA exists completely in the monomer form, and from the absorbance-concentration plot, the molar absorption coefficient of the first overtone of the NH stretching mode has been obtained. The degree of dissociation and dissociation equilibrium constant K have been calculated for the solutions at various concentrations, and then they have been employed to evaluate the thermodynamic parameters (ΔH, ΔS, and ΔG) for the dissociation of hydrogen-bonded NMA into its monomeric species. The results show that the contribution of the entropy (ΔS) to the standard free energy (ΔG) becomes larger with a reduction in the concentration. It seems very likely that in the dilute solutions shorter-chain oligomers exist predominantly, and they easily dissociate into the monomeric species.