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

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.

Exploring the Origin of the Generalized Anomeric Effects in the Acyclic Nonplanar Systems

Contrary to the published conclusions in the literature concerning the origin of the generalized anomeric relationships in open-chain nonplanar systems, its origin has remained an open question. In order to explore the origin of the generalized anomeric relationships in open-chain nonplanar systems, we assessed the roles and contributions of the effective factors on the conformational properties of methyl propargyl ether (1), methyl propargyl sulfide (2), and methyl propargyl selenide (3) by means of the G3MP2, CCSD(T), MP2, LC-ωPBE, and B3LYP methods and natural bond orbital (NBO) interpretations. We examined the contributions of the hyperconjugative interactions on the conformational preferences of compounds 1-3 by the deletions of the orbitals overlapping from the Fock matrices of the gauche- and anti-conformations. The trend observed for energy changes in the Fock matrices justify the variations of the gauche-conformations preferences going from compound 1 to compound 3, revealing that the hyperconjugative interactions are solely responsible for the generalized anomeric relationships in compounds 1-3. Accordingly, the conclusions published in the literature concerning the origin of the generalized anomeric effect in the acyclic nonplanar compounds should be revised by these findings. The Pauli exchange type repulsions (PETR) are in favors of the gauche-conformations and the variations of the PETR differences between the gauche- and anti-conformations of compounds 1-3 correlate well with their gauche-conformations preferences, revealing that the generalized anomeric relationships in compounds 1-3 have also the Pauli exchange-type repulsions origin. The resemblance between the preorthogonal natural bond orbitals (that are involved in the hyperconjugative interactions) and their corresponding molecular orbitals have been investigated.

Assessing the effective factors affecting the conformational preferences and the early and late transition states of the unimolecular retro-ene decomposition reactions of ethyl cyanate, ethyl thiocyanate and ethyl selenocyanate

The variations of Δ[(HCGAE(X₃–C₄weakening) – HCGAE(X₃–C₄strengthening)] parameters correlate well with the variations of the retro-ene decomposition reactions barrier heights going from compound1to compound3.