Baishya Bikash, Prabhu Uday Ramesh, Suryaprakash N
Solid State and Structural Chemistry Unit and NMR Research Centre, Indian Institute of Science, Bangalore 560 012, India.
J Phys Chem B. 2007 Nov 1;111(43):12403-10. doi: 10.1021/jp074873s. Epub 2007 Oct 11.
The differences in chemical shift anisotropies, dipolar couplings, and quadrupolar couplings of two enantiomers in the chiral liquid crystalline media are employed to visualize enantiomers. In spite of the fact that proton has high magnetic moment and is abundantly present in all the chiral molecules, 1H NMR is not exploited to its full potential because of severe overlap of unresolved transitions arising from long- and short-distance couplings. Furthermore, the two spectra from R and S enantiomers result in doubling of the number of observable transitions. The present study demonstrates the application of the selectively excited homonuclear double quantum (DQ) coherence correlated to its single quantum coherence of an isolated methyl group in a chiral molecule. The DQ dimension retains only the passive couplings within the protons of the methyl group while the long-distance passive couplings are refocused, removing the overlap of central transitions, and each enantiomer displays a doublet instead of a triplet unlike in regular selective refocusing experiment. The doublet separation being different for each enantiomer results in their discrimination. The cross section taken along the single quantum dimension pertaining to each transition in the DQ dimension provides the one-dimensional spectra for each individual enantiomer with the complete removal of the overlapped transitions from the other enantiomer. The experiment is robust, the pulse sequence is easy to implement, and the methodology has been demonstrated on different chiral molecules.
在手性液晶介质中，利用两种对映体在化学位移各向异性、偶极耦合和四极耦合方面的差异来可视化对映体。尽管质子具有高磁矩且大量存在于所有手性分子中，但由于长程和短程耦合产生的未解析跃迁严重重叠，1H NMR并未得到充分利用。此外，R和S对映体的两个光谱导致可观测跃迁数量翻倍。本研究展示了选择性激发的同核双量子（DQ）相干与其手性分子中孤立甲基的单量子相干相关的应用。DQ维度仅保留甲基质子内的被动耦合，同时长程被动耦合被重新聚焦，消除了中心跃迁的重叠，并且与常规选择性重新聚焦实验不同，每个对映体显示为双峰而非三重峰。每个对映体的双峰间距不同导致了它们的区分。沿与DQ维度中每个跃迁相关的单量子维度获取的截面为每个单独的对映体提供了一维光谱，完全消除了来自另一个对映体的重叠跃迁。该实验稳健，脉冲序列易于实施，并且该方法已在不同的手性分子上得到验证。
