Studies of the rotational barrier energy of the amide bond using quantum computing and nuclear magnetic resonance (NMR) are focused mainly on its use as a model of the peptide bond. The results of these studies are valuable not only in terms of the fundamental conformational properties of amide bonds, but also in the design of molecular machines, which have recently attracted interest. We investigate the fluxionality of the amide and enamide bonds of compound 3-[(E)-(dimethylamino)methylidene]-1,1-dimethylurea using advanced dynamic NMR experiments and a theoretical evaluation of the density functional theory (DFT) calculation. The dynamic NMR study shows restricted rotation around the amide group (16.4 kcal/mol) and a very high barrier around the enamine group (18.6 kcal/mol). In a structurally similar compound, (E)-3-(dimethylamino)-N,N-dimethylacrylamide (N atom is replaced by CH), the amide barrier is 12.4 kcal/mol and the enamine barrier is 11.7 kcal/mol. The DFT studies of both compounds reveal the electronic origin of this phenomenon. Theoretical calculations reveal the origin of the higher enamine barrier. The better delocalization of the lone pair of electrons on the end nitrogen atom into the antibonding orbital of the neighboring C–N double bond leads to the better stabilization of the ground state, and this leads to a greater increase in the enamine barrier.

中文翻译：

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结构小变，灵活性大变


使用量子计算和核磁共振（NMR）对酰胺键的旋转势垒能的研究主要集中在其作为肽键模型的用途。这些研究的结果不仅在酰胺键的基本构象性质方面很有价值，而且在最近引起人们兴趣的分子机器的设计方面也很有价值。我们利用先进的动态核磁共振实验和密度泛函理论 (DFT) 计算的理论评估，研究了化合物 3-[(E)-(二甲基氨基)亚甲基]-1,1-二甲基脲的酰胺键和烯酰胺键的流动性。动态 NMR 研究显示，酰胺基团 (16.4 kcal/mol) 周围的旋转受到限制，而烯胺基团 (18.6 kcal/mol) 周围的势垒非常高。在结构相似的化合物（E）-3-（二甲基氨基）-N,N-二甲基丙烯酰胺（N原子被CH取代）中，酰胺势垒为12.4 kcal/mol，烯胺势垒为11.7 kcal/mol。两种化合物的 DFT 研究揭示了这种现象的电子起源。理论计算揭示了较高烯胺势垒的起源。末端氮原子上的孤对电子更好地离域到邻近的C-N双键的反键轨道中，导致基态更好的稳定，这导致烯胺势垒的更大增加。