Evaluation of Y- versus linear-delocalization and the role of charge alternation in trimethylenemethane dianion and butadiene dianions

Experimental results and theory have generally been taken to indicate that cross-conjugated trimethylenemethane dianion is more stable than isomeric linear butadiene dianions, engendering the theoretical concepts of Y-aromaticity and stabilizing topological charge alternation. The viabilities of the concepts are tested for these dianions by using RHF ab initio calculations with a series of polarization and diffuse function basis sets. At high levels of theory the relative energies of trimethylenemethane and butadiene dianions have changed to the extent that an explicit preference for the dianion Y-topology is no longer supported. Charge alternation also does not seem to be a dominating effect in stabilizing the C₄H₆^-2 species. The present results indicate that neither factor is a significant stabilizing structural principle for the isolated dianions.