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Experimental Evidence for the Breakdown of the Karplus Relationship for 3J(13C, 1H) in 1H—C—C=13C Systems

Vicinal 1H, 13C coupling constants were measured for rigid unsaturated 1H—C—C=13C molecular frameworks at natural abundance. As a general trend, an empirical range of 4.5–6.5 Hz is observed independent of the value of the dihedral angle between the CH bond and the plane of the double bond. Furthermore, through‐bond and through‐space substituent effects also influence the magnitude of the coupling constants in a similar way to that known for saturated systems. In summary, two different additive transmission mechanisms take part in these coupling paths depending on the orientation of the π‐system with respect to the allylic bond. The experimental evidence presented demonstrates the breakdown of the Karplus rule in this particular structural fragment. © 1997 by John Wiley & Sons, Ltd.