The Importance of the Pauli Exchange-Type Repulsions and Hyperconjugative Interactions on the Conformational Properties of Halocarbonyl Isocyanates and Halocarbonyl Azides

Abstract

To gain further insight into the origin of the anomeric relationships in planar open-chain (acyclic) compounds, we examined the effects of the hyperconjugative generalized anomeric effect (HC-GAE), Pauli exchange-type repulsion (PETR), the electrostatic model associated with the dipole–dipole interactions (EM-DDI), and the attractive electrostatic interactions (AEI) between the natural atomic charges (NACs) on the conformational properties of halocarbonyl isocyanates [halogen = F (1), Cl (2), Br (3)] and halocarbonyl azides [halogen = F (4), Cl (5), Br (6)] by means of G3MP2, CCSD, MP2, and B3LYP methods with the 6–311+G** basis set on all atoms and natural bond orbital interpretation. Importantly, the deletions of the through bond LPN3→σ*C2–X6 hyperconjugative interactions from the Fock matrices of the cis- and trans-conformations lead to the increase of the cis-conformations’ stability compared with their corresponding trans-conformations going from compound 1 to 3 and from compound 4 to 6, revealing the determining effects on the conformational preferences in compounds 1–3 and 4–6. Essentially, the effects of the through space (LPN3→σ*C4–O5 and LPNα→π*Nβ=Nω, respectively) hyperconjugative interactions on the conformational preferences in the isocyanate (1–3) and azide compounds (4–6) are negligible. The EM-DDI fails to account for the conformational preferences in compounds 2, 3, 5, and 6. Therefore, the generalized anomeric relationships in compounds 1–3 and 4–6 result from the cooperative effects of the HC-GAE and PETR. The variations of the AEIs revealed their opposite effects on the trend observed for the conformational preferences in compounds 1–3 and 4–6. Contrary to the usual assumption, the much larger barrier heights of the rotation around the C2–N3 bonds in the azide compounds (4–6) compared with those in the isocyanate compounds (1–3) result from the exchange components and have no hyperconjugative origin.