A computational study of bond dissociation enthalpies in haloethenes

Carbon-hydrogen bond dissociation enthalpies (BDEs) were computed for all haloethenes, C2H4−nXn (n=0-3, X=F, Cl, Br, I), at the B3LYP/6-311+G(3df,2p) level using isodesmic reactions. It was found that C-H bond strengths in the monohaloethenes varied substantially, by as much as 18 kJ mol−1, dependent upon the bond's stereochemical position relative to the halogen. BDEs in the dihaloethanes varied in the order CX2CH-H>(E)-CHXCX-H>(Z)-CHXCX-H. Trends in the computed bond enthalpies were discussed and explained on the basis of relative steric repulsions and hyperconjugative delocalization interactions, as determined from Natural Bond Orbital analysis.