Persistence of Paratropic Ring Currents in Nonplanar, Tub-Shaped Geometries of 1,3,5,7-Cyclooctatetraene

Surprisingly, the anti-aromatic paratropic ring current of the planar D4h transition state of 1,3,5,7-cyclooctatetraene (COT) survives almost all the way to the equilibrium tub-shaped D2d COT structure (plane-to-plane distance, d = 0.76 A); the current vanishes at ca. 80% of the geometric change and ca. 90% of the energy change from planar to tub geometry

An ab initio Valence Bond Study on Cyclopenta-Fused Naphthalenes and Fluoranthenes

To probe the effect of external cyclopenta-fusion on a naphthalene core, ab initio valence bond (VB) calculations have been performed, using strictly atomic benzene p-orbitals and p-orbitals that are allowed to delocalize, on naphthalene (1), acenaphthylene (2), pyracylene (3), cyclopenta[b,c]-acenaphthylene (4), fluoranthene (5), and cyclopenta[c,d]fluoranthene (6). For the related compounds 1-4 and 5,6 the total resonance energies (according to Pauling's definition) are similar. Partitioning of the total resonance energy in contributions from the possible 4n+2 and 4n pi-electron conjugated circuits shows that only the 6 pi-electron conjugated circuits (benzene-like) contribute to the resonance energy. The results show that cyclopenta-fusion does not extend the pi system in the ground state; the five-membered rings act as peri-substituents. As a consequence, the differences in (total) resonance energy do not coincide with the differences in thermodynamic stability. Notwihstanding, the relative energies of the Kekule structures can be estimated using Randic's conjugated circuits model