Abstract
While the simple PPP model accounts for the absolute signs of low-energy π∗ transitions in cyclic π-electron chromophores (numerous new examples of application to heterocycles are given), its greatest contribution lies in the formulation of simple general rules. The previously known mirror-image pairing theorem for charged alternant hydrocarbons and the soft-hard rule for substituent and heteroatom effects on cyclic π-systems are now complemented by a ΔHOMO-ΔLUMO rule which permits prediction of the absolute sign of the lowest energy B term in cyclic π-systems which are formally related to an annulene containing 4N+2 electrons.

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