Stereoelectronic contributions to long-range 1H-1H coupling constants
The contribution of stereoelectronic interactions to NMR coupling constants 3JHH and 4JHH 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 c...
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| Autores principales: | , |
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2002
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| Acceso en línea: | https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_10895639_v106_n34_p7834_Sproviero http://hdl.handle.net/20.500.12110/paper_10895639_v106_n34_p7834_Sproviero |
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| Sumario: | The contribution of stereoelectronic interactions to NMR coupling constants 3JHH and 4JHH 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 H1-C-C-C-Hanti moiety of propane, 4JHH has important contributions from vicinal interactions that include the anti proton while in the H1-C-C-C-Hgauche moiety the main contributions are vicinal interactions that include H1. In alkene fragments, vicinal interactions that involve the π orbitals are the most important contributions to the couplings. Sigma vicinal interactions, which include orbitals 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*(Ccyclopropane-Ccyclopropane) and σ(Ccyclopropane-Ccyclopropane) → 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 Ccyclopropane-Ccyclopropane bond. For the anti cyclopropyl proton, vicinal interactions of the form σ(C-Hanti) → σ*(Ccyclopropane-C) and σ(Ccyclopropane-C) → σ*(C-Hanti) 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 accounts for the overall angular variation of 4J. |
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