The norbornene mystery revealed.

The long-standing mystery surrounding the peculiar electronic structure of norbornene and its derivatives is revealed by the direct comparisons of their properties with those of reference analogues with localized double bonds (i.e. Lewis structures). Our block-localized wavefunction analysis coupled with the computations of NMR parameters evidently supports the manifestation of a density redistribution toward an incipient retro-Diels-Alder reaction.