The aromaticity and antiaromaticity of dehydroannulenes

The molecular structures and the magnetic properties of hexadehydro[12]annulenes and dodecadehydro[18]annulenes have been studied at ab initio and density-functional levels. The calculations show that the unsubstituted dehydro[12]annulene sustains a paratropic ring current in an external magnetic field, while the ring current for the unsubstituted dehydro[18]annulene is diatropic. Since the induced ring current is one measure of the molecular aromaticity, dehydro[18]annulene can be considered aromatic and dehydro[12]annulene antiaromatic. Fused benzene or cyclobutadiene rings destroy both the antiaromaticity and the aromaticity of the dehydroannulenes, while the substitution of the six hydrogens by ethinyl groups affects the aromatic properties of the dehydroannulenes only slightly. The calculations show that the bond alternation is smaller for molecules sustaining large diatropic or paratropic ring currents than for nonaromatic molecules. The ring-current contributions to the 1H NMR chemical shieldings have been estimated for the molecules studied.