8.17:
¹H NMR: Long-Range Coupling
Coupling interactions of nuclei across four or more bonds are usually weak and not observed in spectra.
However, multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, allylic and homoallylic couplings are seen across four or five bonds, respectively, where spin information is communicated via σ–π overlap.
Here, coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
In alkynes, propargylic and homopropargylic couplings are observed across four and five bonds.
Here, 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.
However, meta coupling interactions are generally attributed to σ-electron mechanisms and the zigzag configuration.
Even in the absence of π bonds, protons related by a planar zigzag or W-pathway in rigid molecules can exhibit long-range coupling.
Here, the overlapping minor lobes of the C–H σ molecular orbitals transmit spin information.
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.