Valence bond description of spin properties of pi-conjugated systems

The spin properties of the low energy states of various pi-electron systems with conjugated double bonds are investigated using the valence bond (VB) method with overlap enhanced atomic orbitals (OEAO's) relying on the semiempirical Pariser-Parr-Pople (PPP) model Hamiltonian. It is found that the assumption of the OEAO transferability from one system to another represents a very good approximation, regardless of the topological character of the pi-system or of the spin of the state considered. Truncating the covalent (or maximally-covalent) VB subspace to only a few representative structures also provides excellent results, even when the quantitative singlet-triplet (or doublet-quartet) splittings between the lowest energy states of different multiplicity are considered.