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 tha t the energy surface for inversion of the amino group in piperidine is drastically modified by the presence of electron acceptor groups in t he 4-position of the ring. The efficiency of through-bond interaction (TBI) between the nitrogen donor and the electron acceptor depends str ongly on the conformation. Along the nitrogen inversion pathway, stabi lization occurs in particular in the region between the transition sta te and the axial conformer, whereas the equatorial conformation is les s influenced by TBI. Semiempirical calculations do not appear useful i n 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 TB I appear smaller when a more extensive basis set (6-31G) is used, but electron correlation amplifies them again. The calculations corrobora te experimental findings concerning the effects of through-bond intera ction on molecular structure and energetics, although the agreement be tween theory and experiment is not quantitative.