双碳醇水溶液的1H NMR实验与理论分析
Experimental and Theoretical Analysis of 1H NMR on Double-Carbon Alcohol Aqueous Solutions

采用核磁共振氢谱（1H NMR）和量子化学（QC）方法研究不同温度下乙醇水溶液和乙二醇水溶液中醇与水之间的相互作用。观察实验结果发现两种醇水溶液中水质子的化学位移呈现两种不同的变化趋势。随着含水量的增加，乙醇（ET）水溶液中水质子化学位移急剧降低，而乙二醇（EG）水溶液中水质子的化学位移缓慢增加。两种醇水溶液中的羟基质子随着浓度的增加，其共振峰移向低场。不同温度下随着浓度的增加两种醇水溶液的烷基质子共振峰单调的移向低场。几何结构优化结果表明醇羟基质子与水质子之间氢键的形成弱化了醇中O-H键，从而导致其键长增加。值得注意的是在相同的极化作用和扩散作用下采用密度泛函理论（DFT）（B3LYP）计算得到的ET和EG的C-H键，C-C键和O-H键的键长值大于采用HF理论计算得到的结果。与此相反的是采用HF理论得到的ET和EG的O-H…O键强度大于采用DFT（B3LYP）理论得到的结果。几何构型优化结果与实验结果相吻合。在NMR化学位移的计算中，就文中所提到的理论水平而言，DFT要优于HF。而对于同一理论，其基组越大，计算值越接近实验值。
In this study, 1H nuclear magnetic resonance (NMR) measurements and quantum chemistry (QC) studies of ethanol (ET)-water mixtures and ethylene glycol (EG)-water mixtures are carried out at different temperatures to discuss the interactions between water and the alcohols present in the mixtures. From 1H NMR spectra, it is observed that the chemical shift of the water proton shows two different trends in the ET-water mixtures and the EG-water mixtures. With increasing water concentration, the water proton chemical shift decreases dramatically for ET-water mixtures, while the chemical shift increases slowly for EG-water mixtures. The alcohol hydroxyl proton resonance peaks of both ET and EG shift to lower field with decreasing water concentration. It is found that the resonance peaks of all alkyl protons shift monotonically to low field with increasing alcohol concentration at different temperatures. The geometry optimization results indicate the formation of H-bonds between the water molecules and the hydroxyl groups of the alcohols alongside the weakening of O-H bonds in the alcohols, which results in an O-H bond length decrease. It is interesting to note that the bond length values computed for C-C, C-H and O-H bond in both ET and EG are larger when calculated at the density functional theory (DFT) (B3LYP) level than when calculated using Hartree-Fock (HF) level of theory with the same polarization function and diffusion function. However, the O-H…O H-bond computed at HF level of theory is stronger than that calculated at DFT level of theory. The theoretical results are in good agreement with the experimental ones. In the calculation of NMR chemical shift, DFT(B3LYP) is better than HF, which implies that for the same method, the larger the basis sets are, the more accurate are the calculated values