The destabilization of pi-electron delocalization in cyclopentadiene - A method of separating out of its pi, pi-like and sigma fragment molecular orbitals

A procedure for separating out of pi, pi-like and sigma fragment molecular orbitals (FMO) was established, and it provides for cyclopentadiene a compl ete FMO basis set in which all FMOs have correct electron occupancies. Base d on our new method, the Morokuma energy partition provides, in the framewo rk of ab initio SCF-MO computation at the STO-3G level, the various pi and sigma energies associated with the inter- and intrafragment interactions, T he pi-electronic delocalization is destabilization in cyclopentadiene, and its delocalized pi-system is also destabilized. In the case of benzene, its QMRE is stabilizing. However, it is the sigma-framework of benzene, rather than the pi-system itself, to be strongly stabilized, revealing the benzen e is sigma aromatic, It should be emphasized that the sigma-pi space intera ctions, expressed in terms of the Coulomb J(sigma pi) and exchange K-sp int egrals, have significant effects on the molecular behaviour.