WHY IS A SIMPLE N-ALKYL BRIDGE MORE EFFICIENT THAN A POLYNORBORNYL BRIDGE AT MEDIATING THROUGH-BOND COUPLING

NBO analyses have been conducted on a series of polynorbornyl dienes and alpha,omega-divinyl alkanes in an attempt to delineate the destructive interference mechanisms that cause through-bond coupling in the pi manifold to display a stronger distance dependence in the former series than in the latter. It appears that two principal mechanisms are responsible for the destructive interference in the polynorbornyl dienes. These are the ''cross-links'' mechanism, in which interference is caused by the C-C bonds joining the relays, and the ''cross-talk'' mechanism, in which the interference occurs by way of direct, through-space interactions between the relays. Of these two mechanisms, the ''cross-talk'' mechanism is found to be more important. The bridge C-H bonds are found to have a negligible effect upon the electronic couplings in the polynorbornyl dienes, although they have a significant influence in the divinyl alkanes.