Long-distance effect on one-bond C-C scalar couplings upon introduction of a double bond in bicyclic systems

Anomalous variations of one-bond C-C scalar coupling constants are observed in going from trinorbornane-to trinorbornene-like structures. Most notably, a IO-Hz increase is observed in the coupling constant involving the C-atoms of the ethano branch facing the C=C bond. An effect of about half the size is characteristic of higher homologues (bicyclo[2.2.2]octenes), but no such effect is observed for monocyclic molecules.