Modern Valence-Bond Descriptions of Polycyclic Fused Aromatic Compounds Involving Cyclopropenyl Rings

The feasibilities and electronic structures of five ten-π-electron fused conjugated molecules involving cyclopropenyl rings are explored using second-order Møller-Plesset perturbation theory (MP2), spin-coupled (SC) and complete-active-space self-consistent-field (CASSCF) wavefunctions, in the cc-pVTZ basis. All five fused conjugated molecules are predicted to have rigid planar ground state geometries of C 2v or D 2h symmetry and large dipole moments (if not of D 2h symmetry). The compact ground state SC(10) wavefunctions with ten active orbitals for these molecules are found to be of comparable quality to the respective CASSCF(10,10) constructions, but much easier to interpret. The analyses of the ground state SC(10) wavefunctions for all five fused conjugated molecules reveal resonance patterns which indicate that all of these molecules are aromatic in their electronic ground states; on the other hand, the SC(10) approximations to the first singlet electronic excited states are found to exhibit “antiresonance” which suggests that each of the five molecules switches from aromatic to antiaromatic upon vertical excitation from the ground state to its first singlet excited state. Ring strain prevents the formation of a fused structure involving three cyclopropenyl rings and a cycloheptatrienyl ring; the alternative stable dehydro compound which resembles m-benzyne is shown, using a SC(12) wavefunction, to involve a weak σ bond between the dehydro centres.