Trimethylenemethane dianion and the controversial notion of “y-aromaticity”

The Y-shaped trimethylenemethane dianion (TMM^2-) has been studied by RHF ab initio calculations at the 6-31G level. The planar D_3h form was found to be the most stable conformation. The barriers for 1, 2, and 3 rotations were found to be 21.7, 60.6, and 159.4 kcal/mol, respectively. Moreover, the planar D_3hTMM^2-form was found to be more stable than the isoelectronic, linearly delocalized butadiene dianions by about 30 kcal/mol. The alternation of charges (as calculated on the basis of Mulliken population analysis) was found to be more pronounced than in related systems. The notion of the Y-aromaticity is considered and vitalized on the basis of these data. The unstable character of the triply rotated TMM^2- was found to be appropriately termed “Möbius antiaromaticity” since the destabilizing delocalization is pericyclic.