Vinylcyclopropane resonance energy

Thus the rotation of the allylic system out of the plane (required before ring closure can occur) results in the reduction of the available allylic resonance energy by about 5 kcal mole". This particular steric requirement does not apply to the isomerizations to the dienes, and these have much the same values as for vinylcyclopropane itself, viz. ... 

A final correction is needed because the vinylcyclopropane units in 27 or 10 do not adopt the stable trans forms but less stable gauche forms. This leads to a reduction of the homoconjugative resonance energy by another 2 kcal mol-1. The final resonance energy is 9.7 kcal moT1, clearly indicating the homoantiaromatic character of 27. 

The activation energy is only 7 kcal/mol less than that for the vinylcyclopropane rearrangement itself, consistent with roughly 20 kcal/mol for the pentadienyl radical resonance energy, and suggests that the reaction is not concerted. 

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