Norbornadiene, through-space interactions

In norbornadiene, through-space interaction between the Ji-orbitals is more important than any through-bond interaction. In (19) and (20) an additional interaction with the diene is possible. In Figure 1, vertical ionization potentials of the appropriate jc-orbitals are shown similar effects are observed although the additional methyl groups in (20) result in a displacement of... 

No discussion about strained-ring radical cations would be complete without the valence isomers quadricyclane (15 +) and norbornadiene, (16 +) 15 features two adjacent rigidly held cyclopropane rings, whereas 16 contains two ethene n systems well suited to probe through-space interactions.Molecular orbital considerations suggest the antisymmetric combination of the ethene n orbitals (16) or cyclopropane Walsh orbitals (15) as respective HOMOs of the two parent molecules. The radical ions have different state symmetries and their SOMOs have different orbital symmetries. 

The through-space interaction of the two n bonds of norbornadiene was presented in Chapter 3 as exemplifying a four-electron, two-orbital interaction. The interaction of the nonconjugated n bonds of 1,4-cyclohexadiene cannot be treated in the same way ... 

For example, norbornadiene does not possess a stability that is significantly different from that of a diene without any through-space interactions. Its heat of formation can be... 

The heuristic success of the concepts of through-space and through-bond interactions introduced by Hoffmann200 made it desirable to link it to many electron procedures, at least on the level of semiempirical or ab initio SCF calculations. Such a treatment, which had been proposed some time ago204, has been reviewed in detail elsewhere173. For this reason we shall only sketch the essential steps involved, using norbornadiene 75 as an example. 

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