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 a nd alpha,omega-divinyl alkanes in an attempt to delineate the destruct ive interference mechanisms that cause through-bond coupling in the pi manifold to display a stronger distance dependence in the former seri es than in the latter. It appears that two principal mechanisms are re sponsible for the destructive interference in the polynorbornyl dienes . These are the ''cross-links'' mechanism, in which interference is ca used by the C-C bonds joining the relays, and the ''cross-talk'' mecha nism, 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.