N’-苄基酰腙分子的氮-氮键旋转位阻及分子构象

采用动态核磁共振波谱(DNMR)和密度泛函理论(DFT)对N'-苄基酰腙化合物进行构象研究. 实验和理论计算表明, 1H NMR图谱中三组不同质子的双峰裂分是由N—N键旋转位阻造成的, 而这三个双峰裂分的化学位移差异随温度升高而减小. 通过模拟化学位移差异与温度的关系, 得到了交换速率常数, 采用Eyring方程计算出N—N键旋转位阻. 提出顺式和反式共存的模型来分析酰胺质子信号分裂的原因, 并利用DFT计算得出优化的异构体构象及其最低能量. 端甲基质子和次甲基质子信号裂分也来源于N—N键旋转受阻. N'-苄基酰腙通过缩合反应转变成1,3,4-二唑化合物, 消除了甲基空间取向的差异, 其信号变为单峰.

N-N Rotational Barriers in N'-arylideneacetohydrazide and Molecular Conformation

A molecular conformation study of N'-arylideneacetohydrazide compounds was performed using dynamic NMR (DNMR) spectroscopy and density functional theory (DFT). Three groups of double peak patterns in the 1H NMR spectra were observed, which was the result of rotational hindrance of the N—N bond. The difference in the chemical shift of these peaks decreased with increasing temperature. The exchange rate constants were obtained through simulation of the relationship between the difference in chemical shift and temperature. The energy barriers for rotation of the N—N bond were calculated according to Eyring's equation. A model of coexisting E and Z forms of the N—N bond has been suggested to explain the separation of the NH protons in the NMR spectrum. DFT were carried out to optimize the conformational isomers with minimum energies. Peak separations in the methyl and azomethine signals were also found to originate from the rotation hindrance of the N—N bond. After N'-acylideneacetohydrazides were converted to 1,3,4-oxadiazole compounds, the proton signal of the methyl group appeared as a single peak.