EFFECTS OF THROUGH-BOND ELECTRON DONOR-ACCEPTOR INTERACTION ON THE NITROGEN INVERSION POTENTIAL IN PIPERIDINE-DERIVATIVES - A QUANTUM-CHEMICAL STUDY

Ab initio and semiempirical quantum-chemical calculations indicate that the energy surface for inversion of the amino group in piperidine is drastically modified by the presence of electron acceptor groups in the 4-position of the ring. The efficiency of through-bond interaction (TBI) between the nitrogen donor and the electron acceptor depends strongly on the conformation. Along the nitrogen inversion pathway, stabilization occurs in particular in the region between the transition state and the axial conformer, whereas the equatorial conformation is less influenced by TBI. Semiempirical calculations do not appear useful in assessing through-bond interaction. Low-level ab initio Hartree-Fock calculations (3-21G basis set) are a much more sensitive probe of the effects of TBI, but their accuracy is questionable. The effects of TBI appear smaller when a more extensive basis set (6-31G*) is used, but electron correlation amplifies them again. The calculations corroborate experimental findings concerning the effects of through-bond interaction on molecular structure and energetics, although the agreement between theory and experiment is not quantitative.