Ring currents that survive bond alternation in constrained 8π and 6π monocycles

Current density maps are computed at the ipsocentric CTOCD-DZ/6-31G**//RHF/6-31G** level for angle-constrained planar 1,3,5,7-cyclooctatetraenes (COT) and benzene. Constraint of α(CCH) angles to 90° in D4h COT (3b) leads to endo-4 and exo-4 valence isomers. The exo structure, with CH bonds perpendicular to long sides of the octagon, is lower in energy by 202 kJ/mol and has stronger bond alternation (ΔR 0.265 Å). However, endo-4, exo-4 and COT 3b at its best planar geometry all sustain intense paratropic ring currents, attributed to the rotational character of the HOMO–LUMO transition, and consistent, mutatis mutandis, with the diatropic current in D3h benzene 5.