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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Stereoelectronic Contributions to Long-Range ~1H-~1H Coupling Constants
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Stereoelectronic Contributions to Long-Range ~1H-~1H Coupling Constants

机译：远距离〜1H-〜1H耦合常数的立体电子贡献

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摘要

The contribution of stereoelectronic interaction to NMR coupling constants ~3J_(HH) and ~4J_(HH) has been examined using ab initio calculations and natural bond orbital (NBO) analysis on four model compounds: ethane, propane, propene, and methylcyclopropane. The main stereoelectronic contributions to the couplings originate in three-bond (vicinal) interactions and in through-space interactions. In ethane, besides the main contribution of the σ(C-H) → σ~*(C-H) interaction, other interactions present in the molecule make a decisive contribution to the angular dependence of ~3J. In the H_1-C-C-C-H_(anti) moiety of propane, ~4J_(anti) moiety of propane, ~4J_(HH) has important contributions from vicinal interactions that include the anti proton while in the H_1-C-C-C-H_(gauche) moiety the main contributions are vicinal interactions that include H_1. In alkene fragments, vicinal interactions that involve the π orbitale are the most important contributions to the couplings. Sigma vicinal interactions, which include orbitale corresponding to C-H bonds that involve either of the coupled protons, are crucial to elucidate differences between cisoid and transoid coupling constants. In the case of methylcyclopropane, the most important contributions to the coupling of the syn cyclopropyl H come from the σ(C-H) → σ~*(C_(cyclopropane)-C_(cyclopropane)) and σ(C_(cyclopropane)-C_(cyclopropane)) → σ~*(C-H) vicinal interactions (where the H corresponds to the non-cyclopropyl hydrogen). The concerted effect of several interactions that contribute toward a trend similar to that shown by allyl-vinyl proton couplings is in accordance with a significant π contribution of the C_(cyclopropane)-C_(cyclopropane) bond. For the anti cyclopropyl proton, vicinal interactions of the form σ(C-H_(anti) → σ~*(C_(cyclopropane)-C) and σ_(C_(cyclopropane)-C) → σ~*(C-H_(anti)) are the main contributors to the angular variation of the couplings, similar to what happens to the anti proton in propane. As a whole, the overall behavior of these couplings resembles that of the equivalent proton in propane. In addition, in this case there is not a unique set of interactions which amounts for the overall angular variation of ~4J.
机译：使用从头算和对四种模型化合物：乙烷，丙烷，丙烯和甲基环丙烷的自然键轨道（NBO）分析，研究了立体电子相互作用对NMR偶联常数〜3J_（HH）和〜4J_（HH）的贡献。耦合的主要立体电子贡献源于三键（邻近）相互作用和贯穿空间的相互作用。在乙烷中，除了σ（C-H）→σ〜*（C-H）相互作用的主要贡献外，分子中存在的其他相互作用也对〜3J的角度依赖性起决定性作用。在丙烷的H_1-CCC-H_（反）部分中，丙烷的〜4J_（反）部分中，〜4J_（HH）具有邻位相互作用的重要贡献，其中包括在H_1-CCC-H_（gauche）中的反质子部分的主要贡献是包括H_1的邻近相互作用。在烯烃片段中，涉及π轨道的邻位相互作用是对偶联最重要的贡献。 Sigma邻域相互作用，包括与涉及两个耦合质子的C-H键相对应的轨道，对于阐明顺式和反式耦合常数之间的差异至关重要。对于甲基环丙烷，对顺式环丙基H偶联最重要的贡献来自σ（CH）→σ〜*（C_（环丙烷）-C_（环丙烷））和σ（C_（环丙烷）-C_（环丙烷））→σ〜*（CH）邻位相互作用（其中H对应于非环丙基氢）。几种相互作用的协同效应，其趋势类似于烯丙基-乙烯基质子偶合所显示的趋势，是根据C_（环丙烷）-C_（环丙烷）键的显着π贡献。对于抗环丙基质子，邻位相互作用形式为σ（C-H_（anti）→σ〜*（C_（cyclopropane）-C）和σ_（C_（cyclopropane）-C）→σ〜*（C-H_（反））是偶合器角度变化的主要因素，类似于丙烷中反质子所发生的情况。总体上，这些偶合器的整体行为类似于丙烷中等效质子的行为。在这种情况下，没有唯一的相互作用集相当于〜4J的整体角度变化。

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