Origin of 13C NMR chemical shifts elucidated based on molecular orbital theory: paramagnetic contributions from orbital-to-orbital transitions for the pre-α, α, β, α-X, β-X and ipso-X effects, along with effects from characteristic bonds and groups†

IF 3.9 3区 化学 Q2 CHEMISTRY, MULTIDISCIPLINARY
RSC Advances Pub Date : 2025-01-21 DOI:10.1039/D4RA05980H
Waro Nakanishi, Satoko Hayashi and Keigo Matsuzaki
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引用次数: 0

Abstract

13 C NMR chemical shifts (δ(C)) were analysed via MO theory, together with the origin, using σd(C), σp(C) and σt(C), where C4− was selected as the standard for the analysis since σp(C: C4−) = 0 ppm. An excellent relationship was observed between σd(C) and the charges on C for (C4+, C2+, C0, C2− and C4−) and (C4−, CH22−, CH3 and CH4). However, such a relationship was not observed for the carbon species other than those above. The occupied-to-unoccupied orbital (ψiψa) transitions were mainly employed for the analysis. The origin was explained by the pre-α, α, β, α-X, β-X and ipso-X effects. The pre-α effect of an approximately 20 ppm downfield shift is theoretically predicted, and the observed α and β effects of approximately 10–15 ppm downfield shifts are well reproduced by the calculations, as are the variations in the α-X, β-X and ipso-X effects. Large downfield shifts caused by the formation of ethene (∼120 ppm), ethyne (∼60 ppm) and benzene (∼126 ppm) from ethane and carbonyl (∼146 ppm) and carboxyl (∼110 ppm) groups from CH3OH are also reproduced well by the calculations. The analysis and illustration of σp(C) through the ψiψa transitions enables us to visualize the effects and to understand the δ(C) values for the C atoms in the specific positions of the species. The occupied-to-occupied orbital (ψiψj) transitions are also examined. The theoretical investigations reproduce the observed results of δ(C). The origin for δ(C) and the mechanism are visualized, which allows us to image the process in principle. The role of C in the specific position of a compound in question can be more easily understood, which will aid in the development of highly functional compounds based on NMR.

Abstract Image

基于分子轨道理论阐明13C核磁共振化学位移的起源:轨道到轨道跃迁对前α、α、β、α- x、β- x和ipso-X效应的顺磁贡献,以及特征键和基团†的影响
用MO理论分析了13c核磁共振化学位移(δ(C)),并用σd(C)、σp(C)和σt(C)分析了来源,其中选择C4−作为分析标准,因为σp(C: C4−)= 0 ppm。在(C4+, C2+, C0, C2−和C4−)和(C4−,CH22−,CH3−和CH4)中,σd(C)与C上的电荷有很好的关系。然而,除上述碳种外,没有观察到这种关系。主要采用已占轨道到未占轨道(ψi→ψa)跃迁进行分析。其成因可由pre-α、α、β、α- x、β- x和ipso-X效应解释。理论上预测了约20 ppm下场位移的前α效应,计算结果很好地再现了约10-15 ppm下场位移的α和β效应,以及α- x、β- x和ipso-X效应的变化。计算还很好地再现了由乙烷和羰基(~ 146 ppm)以及羧基(~ 110 ppm)形成乙烷(~ 120 ppm)、乙炔(~ 60 ppm)和苯(~ 126 ppm)引起的大的下场位移。通过ψi→ψa跃迁对σp(C)的分析和说明,使我们能够直观地理解在物质的特定位置上的C原子的δ(C)值。我们还研究了已占轨道到已占轨道(ψi→ψj)的跃迁。理论研究再现了δ(C)的观测结果。δ(C)的起源和机理是可视化的,这使我们能够在原则上对过程进行成像。C在化合物特定位置的作用可以更容易地理解,这将有助于基于核磁共振开发高功能化合物。
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来源期刊
RSC Advances
RSC Advances chemical sciences-
CiteScore
7.50
自引率
2.60%
发文量
3116
审稿时长
1.6 months
期刊介绍: An international, peer-reviewed journal covering all of the chemical sciences, including multidisciplinary and emerging areas. RSC Advances is a gold open access journal allowing researchers free access to research articles, and offering an affordable open access publishing option for authors around the world.
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