The Origin of Cis Effect in 1,2-Dihaloethenes: The Quantitative Comparison of Electron Delocalizations and Steric Exchange Repulsions

Abstract

Abstract The “cis effect” is a phenomenon in which the cis isomer is more stable than the corresponding trans isomer or almost the same stability in some molecules with double bonds. In order to clarify the predominant factor of this cis effect in the 1,2-dihaloethenes (XHC=CHX; 1: X=F, 2: Cl, and 3: Br), the energetic amount of electron delocalizations and steric exchange repulsions were theoretically estimated using the natural bond orbital (NBO) theory at MP2/6-311++G(3df,3pd) level. Two delocalization mechanisms, periplanar hyperconjugations (synperiplanar and antiperiplanar effects) and halogen lone pair delocalizations into the C=C bond antibonding orbitals (LP effect), were found as the cis stabilizing forces, in which the total amount of LP effect was greater than those of periplanar effects, the dominant factor of the cis effect. Moreover, the origin of the cis preference of the LP effect was clearly elucidated with the application of orbital phase theory, i.e., the cyclic orbital interaction was continuous only in the cis isomers of 1–3. The total steric exchange repulsion between two isomers were all trans stabilizing and their energetic gains were 1.26, 16.48, and 23.22 kJ mol−1 for 1, 2, and 3, respectively. These steric forces obviously counteract against cis preferable delocalization mechanisms, especially in compounds with larger halogen atoms, but their amounts are apparently less than those of electron delocalizations (29.82, 40.00, and 34.46 kJ mol−1 for 1, 2, and 3, respectively). Therefore, electron delocalization, not exchange repulsion, has the largest responsibility for the relative energies of 1,2-dihaloethene systems. The importance of this work is the quantitative elucidation of the dominance of delocalization mechanisms over steric effects on the electronic and energetic view of a simple molecular structure.