Experimental Evidence for the Breakdown of the Karplus Relationship for3J (13C, 1H) in 1H - C - C=13 C 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.