EXPERIMENTAL-EVIDENCE FOR THE BREAKDOWN OF THE KARPLUS RELATIONSHIP FOR (3)J(C-13, H-1) IN H-1-C-C=C-13 SYSTEMS

Vicinal H-1, C-13 coupling constants were measured for rigid unsaturat ed H-1-C-C=C-13 molecular frameworks at natural abundance. As a genera l trend, an empirical range of 4.5-6.5 Hz is observed independent of t he value of the dihedral angle between the CH bond and the plane of th e double bond. Furthermore, through-bond and through-space substituent effects also influence the magnitude of the coupling constants in a s imilar way to that known for saturated systems. In summary, two differ ent additive transmission mechanisms take part in these coupling paths depending on the orientation of the pi-system with respect to the all ylic bond. The experimental evidence presented demonstrates the breakd own of the Karplus rule in this particular structural fragment.