A fresh look at aromatic dianions

Isolated dianions of cyclooctatetraene and cyclobutadiene are, despite their aromatic character, unstable with respect to electron loss. Here we present the first ab initio study of the autodetachment lifetimes of several "aromatic" dianions. In particular, we investigate whether aromatic pi systems are associated with an enhanced stability with respect to electron loss and whether cross-conjugated (Y-aromatic) hydrocarbon dianions possess longer lifetimes than their linear-conjugated isomers. Moreover, our results are compared with bound state calculations for these metastable dianions, allowing us to analyze the validity of the latter approach. All considered, hydrocarbon dianions are found to possess surprisingly short autodetachment lifetimes on the order of a few femtoseconds, and therefore only electron scattering techniques seem to be capable of observing these species in the gas phase. The trends of the computed energies and lifetimes are best understood in terms of the mutual Coulomb repulsion of the extra charges. Small gas phase dianions can only be stable if the excess electrons can localize at least at two subgroups of high local electron affinity. Adding both extra electrons into one delocalized pi system leads to a very unstable species, and thus especially those structures associated with pronounced aromaticity are found to show high energies and short lifetimes.