Abstract
The structural changes accompanying the electronic excitation of the antiaromatic biphenylene molecule were studied by semiempirical quantum chemical calculations, using the AM1 hamiltonian with configuration interaction (S 1, S 2, T 1, T 2, X̃ + states). The largest bond length differences of the molecular states investigated were observed within the central four-membered ring. Potential curves of the CC bonds which connect the two benzene rings have been calculated for all molecular states investigated. For the S 0 and S 1 states, potential hypersurfaces, using the two different bond lengths of the central four-membered “cyclobutadiene” subsystem as variables, were presented. Furthermore, the geometry of 1,3-cyclobutadiene in different states (S 0, S 1, T 1, X̃ + states) was investigated with the AM1-CI hamiltonian and compared with recent 6-31G ∗ MC-SCF ab initio results.
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