Theoretica Chimica Acta

, Volume 95, Issue 3–4, pp 67–79 | Cite as

The delocalisation energy of benzene and the non-empirical MO theory

  • Melvyn P. Melrose


Algebraic expressions for the vertical Delocalisation Energy (DE) of benzene are derived from non-empirical MO theory. For comparison with early work in the π-electron approximation, and ultimately with Hückel theory, the results are formulated in terms of a core resonance integral,β, and π-electronic repulsion integrals. All integral values are inferred from the results ofab initio SCF calculations. Two expressions are derived, which refer to two ways of forming the localised π MOs: one where three pairs of adjacent atomic orbitals are selected from a set of six orthogonalised orbitals; and another where a non-orthogonal set of atomic orbitals is used. The first expression is formally similar to an expression originally derived by Pople from a different point of view and with many approximations. This expression gives too large a magnitude for DE when used with anab initio value ofβ. The second expression gives a result much closer to an empirical value of DE and shows that the main reason for DE being about 50% of 2β rather than 2β is the stabilising effect of overlap in the localised structure, and that the less important factor is the inclusion of electronic repulsion.

Key words

Delocalisation energy Non-empirical MO Resonance integral 


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Copyright information

© Springer-Verlag 1997

Authors and Affiliations

  • Melvyn P. Melrose
    • 1
  1. 1.Department of ChemistryKing's College LondonStrandUK

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