False AA'X Spin-Spin Coupling Systems in 13C NMR: Examples Involving Phosphorus and a 20-Year-Old Mystery in Off-Resonance Decoupling
False AA'X Spin-Spin Coupling Systems in 13C NMR: Examples Involving Phosphorus and a 20-Year-Old Mystery in Off-Resonance Decoupling
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13C NMR 中的假 AAX 自旋-自旋耦合系统：涉及磷的示例和非共振解耦中 20 年前的谜团
DOI：
10.1021/ed074p1485
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发表时间：
1997
影响因子：
3
通讯作者：
W. Hersh
中科院分区：
文献类型：
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作者：
W. Hersh
在NMR教科书和期刊文章中，通常将某些ABX自旋系统松散地描述为AA 'X自旋系统，其中AB对中的核由于一个接近12 C而另一个接近13 C而在化学上不等价。虽然化学位移通常是不可区分的，因此对于所有实际目的，这样的系统可以被分析为AA 'X，但存在这样的危险，即当化学位移不相同时，由于假设同位素异构体位移总是不显著，人们可能无法认识到这一事实。对于二磷有机化合物和有机金属化合物，描述了这种假AA 'X光谱的实例，其中使用400 MHz（1H）NMR光谱仪，Δ vPP' = 2.5 - 4 Hz;在较低场强下，由于Δ vPP '的减少，ABX分裂可能预期消失。在马来酸酐的核磁共振谱中，13 C的非共振去偶被证明依赖于倍频功率：在低功率下，系统是ABX，而在高功率下，系统是ABX。
It is common in both NMR textbooks and journal articles to loosely describe certain ABX spin systems, in which the nuclei in the AB pair are rendered chemically inequivalent by proximity of one to 12C and the other to 13C, as AA'X spin systems. While the chemical shifts normally are indistinguishable and so for all practical purposes such systems can be analyzed as AA'X, there is a danger that when the chemical shifts are not the same, one may fail to recognize that fact due to the assumption that the isotopomer shift is always insignificant. Examples of such false AA'X spectra are described for diphosphorus organic and organometallic compounds, in which DeltavPP' = 2.5 - 4 Hz using a 400 MHz (1H) NMR spectrometer; at lower field strength the ABX splitting might be expected to disappear due to the reduction in DeltavPP'. An example involving 13C off-resonance decoupling in the NMR spectra of maleic anhydride was shown to depend on decoupler power: at low power, the system is ABX, while at the high power n...