Peculiarities in the quantum statistics of pi electron systems

The peculiarities in the quantum statistics of pi electron systems an analyzed by comparing pi energies derived for the conventional fermionic (fe) ensemble with so-called hard core bosonic (hcb) energies. hcb ensembles combine fermionic on-site and bosonic intersite properties. They obey the Pauli exclusion principle (PEP) but not the Pauli antisymmetry principle (PAP). In the present study we extend our previous theoretical findings evaluated for one-dimensional (1D) model structures to 2D arrangements realized in hydrocarbon pi molecules. Topological electronic ordering principles which have been derived for 1D order can be transferred to 2D networks. It is demonstrated that pi systems are destabilized with increasing deviations from the hcb boundary. In the annulene series this destabilization occurs in the 4n anti-Huckel rings (n = 1, 2, 3,...). The competition between quantum constraints (PEP, PAP) and the two-electron interaction of pi Hamiltonians on the energy of pi systems is discussed. Numerical results are given for the Pariser-Parr-Pople, Hubbard and Huckel molecular orbital pi Hamiltonians. (C) 1997 Elsevier Science B.V.