8.17: 1H NMR: Long-Range Coupling
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals. Similarly, propargylic and homopropargylic couplings are observed across four and five bonds in alkynes. However, the cylindrical π system ensures orbital overlap in all orientations, making these couplings geometry-independent. Conjugated double bonds enhance long-range coupling, as seen in the para coupling constants of substituted benzenes.
Under certain conditions, long-range coupling is also seen in molecules without π bonds. In rigid molecules, protons related by a planar zigzag or W-pathway can exhibit long-range coupling. Here, spin information is transmitted by the overlapping minor lobes of the C–H σ molecular orbitals. In aromatic systems, meta coupling interactions are generally attributed to the zigzag configuration and σ-electron mechanisms.
