Two-Bond, Carbon-Proton Coupling in Halogenated Ethylenes

The large differences between the two-bond, carbon-proton couplings in cis- and trans-disubstituted ethylenes have been difficult to account fora. As an aid to additional clarification of this problem, we have determined two-bond coupling constants of several halogenated ethylenes which are presented in Table I. The predicted ^(13)C spectrum of vinyl chloride was found to be sensitive to the relative signs of the carbon-proton coupling constants. The polyhaloethylenes exhibited first-order spectra and double resonance was required to obtain the relative signs.