Through-Bond Orbital Coupling, the Parity Rule, and the Design of “Superbridges” Which Exhibit Greatly Enhanced Electronic Coupling: A Natural Bond Orbital Analysis

Abstract

An ab initio molecular orbital study of through-bond (TB) orbital interactions has been carried out on several series of diene hydrocarbons, 2(n)−17(n), in which the double bonds are covalently attached to a variety of rigid saturated hydrocarbon bridges with lengths, n, ranging from four to 17 C−C σ bonds. The resulting TB π+,π- and π*+,π*- splitting energies, ΔE(π) and ΔE(π*), respectively, were obtained at the HF/3-21G level of theory. The distance dependence of ΔE(π) and ΔE(π*) for each type of diene was fitted to the respective exponential decay profiles, ΔE(π) = A exp(−βhn) and ΔE(π*) = B exp(−βen). It was found that both βh and βe were dependent on the nature of the hydrocarbon bridge. For example, βh is found to range from 0.6 per bond for 3(n) to only 0.05 per bond for 7(n) and 8(n). The βh values for the polynorbornane bridge dienes, 2(n), and the hybrid norbornane−bicyclo[2.2.0]hexane bridge dienes, 3(n), are notably larger than that for the divinylalkanes, 4(n), and Natural Bond Orbital (NBO) ...