The peculiarities in the quantum statistics of pi electron systems an
analyzed by comparing pi energies derived for the conventional fermion
ic (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 antisy
mmetry principle (PAP). In the present study we extend our previous th
eoretical 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 ca
n be transferred to 2D networks. It is demonstrated that pi systems ar
e destabilized with increasing deviations from the hcb boundary. In th
e annulene series this destabilization occurs in the 4n anti-Huckel ri
ngs (n = 1, 2, 3,...). The competition between quantum constraints (PE
P, PAP) and the two-electron interaction of pi Hamiltonians on the ene
rgy of pi systems is discussed. Numerical results are given for the Pa
riser-Parr-Pople, Hubbard and Huckel molecular orbital pi Hamiltonians
. (C) 1997 Elsevier Science B.V.