Nuclear magnetic resonance studies of 1,3-butadienes

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Abstract
The treatment of Karplus for the pi-electron contribution to proton-proton coupling constants is extended in this paper to conjugated systems. The equation derived is applied to s-trans and s-cis 1,3-butadienes, and the coupling constants obtained are compared with the experimental values. It is concluded that the five bond coupling constants 5 J tc and 5 J cc (the subscripts c and t refer to the cis or trans orientation of the bonds of the coupling path—see Part I) in the s-trans molecule are due to a pi mechanism, and that for cyclohexadiene the five bond coupling constant has a large contribution from this effect. The results for the four bond couplings are less conclusive, but it is suggested that a pi mechanism is dominant in s-trans dienes, whereas a sigma mechanism dominates for cyclohexadiene.

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