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

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 (), Cl (), Br ()] has remained an open question. To explore the origin of the axial-conformation preferences in compounds -, 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 - 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 -, and justify their corresponding total energy differences. The through-space hyperconjugative interactions between the donor lone pairs of halogen atoms (LPX) and the acceptor antibonding orbitals of H-N bonds [σ* ], LPX → σ* , increase from compound to compound . The inspection of the dipole moments of the parallel C-X and H-N bonds in the axial conformations of compounds - 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 -. Accordingly, the charge-dipole orienting effect associated with the four-center dipole-dipole interactions is a dominant factor in the conformational behaviors of compounds -.