Exploring the Origin of the Axial-Conformation Preferences in the 3-Halopiperidinium Cations: the Importance of the Coulombic Potential Energies.

Abstract

Although there are some published conclusions in the literature concerning the origin of the axial-conformation preference in 3-fluoropiperidinium cations (charge–dipole orientation effect), the origin of the axial-conformation preferences in the 3-halopiperidinium cations [halogen = F (1), Cl (2), Br (3)] has remained an open question. To explore the origin of the axial-conformation preferences in compounds 1–3, we assessed the roles and contributions of the hyperconjugative interactions, the Coulombic electrostatic interactions, the electrostatic model associated with dipole–dipole interactions, and the steric effects associated with the Pauli exchange-type repulsions on the conformational properties of compounds 1–3 utilizing the G3MP2, LC-ωPBE, and B3LYP methods and natural bond orbital (NBO) interpretations. Natural Coulombic potential energies are in favor of the axial conformations of compounds 1–3, and justify their corresponding total energy differences. The through-space hyperconjugative interactions between the donor lone pairs of halogen atoms (LP3X) and the acceptor antibonding orbitals of H–N bonds [σ*(H–N)⊕], LP3X → σ*(H–N)⊕, increase from compound 1 to compound 3. The inspection of the dipole moments of the parallel C–X and H–N bonds in the axial conformations of compounds 1–3 revealed that the variations of their corresponding four-center dipole–dipole interactions correlate well with their corresponding conformational behaviors. The steric effects associated with the Pauli exchange-type repulsions are strongly in favor of the equatorial conformations of compounds 1–3. Accordingly, the charge–dipole orienting effect associated with the four-center dipole–dipole interactions is a dominant factor in the conformational behaviors of compounds 1–3.