Exploring the origin of the cis-configurations preferences in 1,2-dihaloethenes. Assessingvthe roles and contributions of the Pauli exchange type repulsions,vhyperconjugative interactions and bonding orbitals deviations

The origin of the cis-configurations preferences in 1,2-difluoroethene (1), 1,2- dichloroethene (2) and 1,2-dibromoethene (3) were analyzed by means of G3MP2 composite method and long range corrected hybrid-density functional theory (LC-roPBE) with the 6-311+G** basis set on all atoms and natural bond orbital interpretation. In these methods used in this work showed the cis-configurations preferences (compared to their transconfigurations) decrease from compound 1 to compound 3. Based on the results obtained, the deletions of the hyperconjugative interactions from the Fock matrices of the cis- and transconfigurations of compound 1 lead to the increase of the trans-conformations stability (by 6.11 kcal mol-1) compared to its corresponding cis-conformation. Accordingly, the cooperative stabilizations associated with the bent C=C bond paths and total hyperconjugative generalized anomeric effect overcome the destabilizations associated with the exchange component and dipole-dipole interactions, leading to the cis-configuration preference in compound 1. The cis-conformations of compounds 1-3 with large dipole moments are still more stable than their corresponding trans-conformation, implying that the electrostatic model associated with the dipole-dipole interactions do not play a determining role on the anomeric preferences in these compounds. This is important evidence that reveals the isomer with larger molecular dipole moment is not the less stable one which contradicts the published conclusion in the literature.