Unexpected signs of five-bond and six-bond proton spin–spin coupling constants in some acyclic conjugated systems. Through-space couplings.

Abstract

The signs and magnitudes of spin–spin coupling constants over five and/or six bonds in some N-methyl and N,N-dimethyl hydrazones, in methyl vinyl ether, 4-methoxy-3-butene-2-one, 1-methoxy-1,3-butadiene, 1-butene, and in anisole are discussed in terms of coupling mechanisms. In the hydrazones the five-bond couplings between methyl and olefinic protons are negative, while six-bond couplings between protons in methyl groups are positive, a reversal of the expected sign sequence. INDO-MO-FPT calculations are not in agreement with observation, but the disagreement may be a consequence of faulty structural assumptions. The sizeable six-bond couplings imply a negative σ–π contribution to the five-bond constants. The calculated and observed negative couplings over five bonds in methyl vinyl ether and the other molecules above emphasize their dependence on the proximity of the bonds containing the coupled nuclei and are reliable indicators of bond arrangements in these molecules.