Stereospecificity of 1H, 13C and 15N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on 1H chemical shifts
Stereospecificity of 1H, 13C and 15N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on 1H chemical shifts
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DOI：
10.1002/mrc.3828
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发表时间：
2012-07-01
影响因子：
2
通讯作者：
Keiko, Natalia A.
中科院分区：
文献类型：
--
作者：
Afonin, Andrei V.;Pavlov, Dmitry V.;Keiko, Natalia A.
在甲基乙二醛双二甲基腙的13C核磁共振谱中，13C-5信号在从EE到ZE的过程中向高频偏移，而13C-6信号则向低频偏移。令人惊讶的是，尽管CH-N键周围的构型没有改变，但ZE异构体的1H-6化学位移和1J（C-6,H-6）偶联常数明显大于EE异构体。这个悖论可以用ZE同分异构体中的C-H=N分子内氢键来解释，这是在量子化学计算中发现的，包括分子分析中的Bader原子量子理论。这个氢键导致d（1H-6）和1J（C-6,H-6）参数的增加。CH？由于这些光谱参数，屏蔽1H上的N个氢键和单键13C1H偶联使所考虑的化合物的构型分配复杂化。与C(CH3)- n和CH-N基团相连的2-和8-(CH3)2N基团的1H、13C和15N化学位移有显著差异。从头计算表明，8-(CH3)2N基团与p-骨架有效共轭，2-(CH3)2N基团从骨架平面扭曲而失去共轭。因此，N-2和N-8氮孤对向p-框架的电荷转移程度不同，从而影响了1H、13C和15N的屏蔽。版权所有：John Wiley & Sons， Ltd
In the 13C NMR spectra of methylglyoxal bisdimethylhydrazone, the 13C-5 signal is shifted to higher frequencies, while the 13C-6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the 1H-6 chemical shift and 1J(C-6,H-6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the CH-N bond does not change. This paradox can be rationalized by the C-H=N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum-chemical calculations including Bader's quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of d(1H-6) and 1J(C-6,H-6) parameters. The effect of the CH?N hydrogen bond on the 1H shielding and one-bond 13C1H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The 1H, 13C and 15N chemical shifts of the 2- and 8-(CH3)2N groups attached to the C(CH3)-N and CH-N moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8-(CH3)2N group conjugate effectively with the p-framework, and the 2-(CH3)2N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N-2 and N-8 nitrogen lone pairs to the p-framework varies, which affects the 1H, 13C and 15N shieldings. Copyright (C) 2012 John Wiley & Sons, Ltd.