Alternation of Bonds in Polyenes

Our second-order treatment of polyenes led to a picture in terms of localized double and single bonds, the former presumably being stronger this can be further justified by the following argument. 

Rupture of a single bond in an open-chain polyene gives rise to two even AHs e.g., 

Union of these to regenerate the original AH involves only a small second-order change in n energy. The resulting bond will therefore be little stronger than its o component. 

The signs of the coefficients flor and are arbitrary in equation (3.24) it is assumed that we have arranged for their product to be positive. If the product were negative, we would have to set SE = — 2 or o As to ensure that the electrons in RS occupy the lower of the two MOs arising from interaction of the NBMOs see Fig. 3.8. 

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