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 (LP 3 X) and the acceptor antibonding orbitals of H–N bonds [σ* (H–N) ⊕ ], LP 3 X → 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 .