Electron transfer due to through-bond interactions. Study of aliphatic chains

Abstract

The electron transfer capability of systems of the type H 2H-(CH 2) n -NH 2 is studied using ab initio and semi-empirical methods. The coupling energy Δ between the nitrogen lone-pairs may, in the symmetric case, be defined as the energy difference between the symmetric and antisymmetric many-particle states of H 2N-(CH 2) n -NH 2 + where ionization has occurred in the lone-pair orbitals. Δ may be related to the energy splitting between the corresponding orbital energies in the neutral molecule according to Koopmans' theorem. A “broken-symmetry” correction (BSC) both in space and spin is introduced. The two localized unrestricted Hartree-Fock solutions are subject to configuration interaction using transformation to “corresponding orbitals”. In saturated chains H 2N-(CH 2) n -NH 2 +, the lowering of |Δ| amounts to about 25%. Ab initio and semi-empirical results show the same trends in the contributions to Δ. In cases when different contributions cancel, the relative difference may be large. The conformation dependence is studied. For the case of sp 3 lone-pairs, Δ is reduced for the cis case to a small fraction of the trans value and in some cases the sign is changed. The largest Δ is obtained for the all-trans conformation.