New Insights into the Origin of the cis-Configuration Preferences in 1,2-Dihaloethenes: The Importance of the Bonding Orbital Deviations

Abstract

The origin of the preferences for the cis-configurations in 1,2-difluoroethene (1), 1,2-dichloroethene (2), and 1,2-dibromoethene (3) were explored by means of the G3MP2, LC-ωPBE and CCSD(T) methods with the 6–311+G** basis set on all atoms, and natural bond orbital interpretation. On the basis of the results obtained, the cis-configurations preferences decrease in going from compound 1 to compound 3. Effectively, the deletions of the hyperconjugative interactions from the Fock matrices of the cis- and trans-configurations of compound 1 lead to the increase of the trans-conformation stability (by ~6.11 kcal mol−1) compared with its corresponding cis-conformation. However, the Pauli exchange-type repulsion difference between the cis- and trans-configurations of compound 1 is in favour of the trans-configuration (by ~6.25 kcal mol−1), revealing that the stabilization energies associated with the hyperconjugative interactions do not compensate the destabilizations associated with the exchange component and dipole-dipole interactions. Importantly, the C=C bond paths in the cis-configuration of compound 1 are bent in essentially the same direction (towards the C–F bonds), leading to an increased overlap and a stronger C–C bond, whereas the C–C bond paths in the trans-configuration are bent in opposite directions. Accordingly, the co-operative stabilizations associated with the bending of the C=C bond paths (towards the C–F bonds) and total hyperconjugative generalized anomeric effect overcome the destabilizations associated with the exchange component and dipole–dipole interactions, leading to the preference of the cis-configuration in compound 1. The deletions of all the donor–acceptor electronic interactions from the Fock matrices of the cis- and trans-configurations of compounds 2 and 3 lead to the increase of the trans-conformation stabilities compared with their corresponding cis-conformations, revealing the determining impacts of the hyperconjugative interactions on the configurational preferences in compounds 2 and 3.