Abstract

The total π-electron energy E π of an even alternant hydrocarbon can be expressed in terms of the matrix elements H ab of the hamiltonian operator H between the so-called “regular resonance structures” (RRSs). This energy is approximately proportional to the sum Of H ab involving only Kekulé type structures. Quantitative expressions for these matrix elements are given, and the significance of the 4 m and 4 m + 2 cycles is explicitly demonstrated. The Hückel 4 m + 2 rule is a simple consequence of this approach; it can be formulated quantitatively and is derived within the Hückel as well as the Pople approximation.

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