Conformational analysis. Part 35. NMR, solvation and theoretical investigation of rotational isomerism in methyl fluoroacetate and methyl difluoroacetate
Conformational analysis. Part 35. NMR, solvation and theoretical investigation of rotational isomerism in methyl fluoroacetate and methyl difluoroacetate
复制标题
构象分析。
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
R. Rittner
中科院分区：
文献类型：
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作者：
R. J. Abraham;C. Tormena;R. Rittner
报道了氟乙酸甲酯（MFA）和二氟乙酸甲酯（MDFA）的13C核磁共振光谱对溶剂和温度的依赖性，并从构象耦合和能量方面分析了1JCF耦合。密度泛函理论（DFT）计算得到了构象的几何形状，溶剂化理论给出了构象能量与溶剂的依赖关系。在MFA中，B3LYP/6-311+G（d,p）水平上的DFT方法只给出了两个能量相等的顺式（F-C-CO 0°）和反式（F-C-CO 180°）构象的能量最小值。间扭式构象在能面上不是最小的。MFA的FTIR光谱支持这一结果，观察到两个分解的羰基带，其相对强度随溶剂极性的变化而显著变化。仅假设这些形式，通过溶剂化理论分析所观察到的耦合导致顺式和反式构象之间的能量差（Ecis - Etrans）在气相中为0.90 kcal mol - 1，在CCl4和DMSO中分别降至0.41 kcal mol - 1和- 0.71 kcal mol - 1。在MDFA中，DFT计算给出了顺式（H-C-CO 0°）和间扭式（H-C-CO 141.9°）构象的两个最小值，能量差（Ecis - Egauche）为0.2 kcal mol - 1。MDFA的FTIR光谱支持这一结果，在非极性溶剂（CCl4）中观察到两个分辨带，但在中极性和高极性溶剂中羰基吸收表现为单个带。假设只有这两种形式，通过溶剂化理论分析所观察到的耦合导致顺式和间扭式构象之间的能量差（Ecis - Egauche）在气相中为0.0 kcal mol - 1，在CCl4和DMSO中分别增加到0.46 kcal mol - 1和1.12 kcal mol - 1。
The solvent and temperature dependence of the 13C NMR spectra of methyl fluoroacetate (MFA) and methyl difluoroacetate (MDFA) are reported and the 1JCF coupling analysed in terms of the conformer couplings and energies. Density Functional Theory (DFT) calculations were used to obtain the conformer geometries and solvation theory gave the solvent dependence of the conformer energies. In MFA the DFT method at the B3LYP/6-311+G(d,p) level gave only two energy minima for the cis (F–C–CO 0°) and trans (F–C–CO 180°) conformers of ca. equal energy. The gauche conformer was not a minimum in the energy surface. The FTIR spectra of MFA support this result as two resolved carbonyl bands are observed whose relative intensity changes markedly with solvent polarity. Assuming only these forms, the observed coupling when analysed by solvation theory leads to the energy difference (Ecis − Etrans) between the cis and trans conformers of 0.90 kcal mol−1 in the vapour phase, decreasing to 0.41 kcal mol−1 in CCl4 and −0.71 kcal mol−1 in DMSO. In MDFA the DFT calculations gave two minima for the cis (H–C–CO 0°) and gauche (H–C–CO 141.9°) conformers with an energy difference (Ecis − Egauche) of 0.2 kcal mol−1. The FTIR spectra of MDFA support this result as in the non-polar solvent (CCl4) two resolved bands are observed but in solvents of medium and high polarity the carbonyl absorption appears as a single band. Assuming only the two forms, the observed coupling when analysed by solvation theory leads to the energy difference (Ecis − Egauche) between the cis and gauche conformers of 0.0 kcal mol−1 in the vapour phase, increasing to 0.46 kcal mol−1 in CCl4 and 1.12 kcal mol−1 in DMSO.