The ring current in cyclopropane

Mapping of induced current density using the coupled Hartree-Fock “ipsocentric” (CTOCD-DZ/ 6-31G**//RHF/6-31G**) method shows a ring current arising in the σ framework of cyclopropane that is intense, diatropic and annular, and hence is consistent with the numerous literature attributions of σ-aromaticity to this molecule. Localised orbital analysis shows that this current can be attributed to the set of three carbon–carbon bonds. In a similar analysis, the four carbon–carbon bonds of (planar constrained) cyclobutane are found to give rise to a central paratropic current. The maps are consistent with the view that cyclopropane is strongly σ-aromatic, cyclobutane is weakly σ-anti-aromatic, and the larger cycloalkane systems are essentially σ-non-aromatic.