Squeezing carbon dimers into the smallest fullerenes: DFT prediction of new carbon clusters containing double hypercoordinate carbons

The viabilities of squeezing carbon dimers into the smallest fullerenes, C20 and C24, were investigated based on density functional theory computations. Each of the resulting endofullerenes, C2@C20 and C2@C24, contains double hypercoordinate carbons bonded with each other. These novel endofullerenes provide the first examples of hypercoordinate carbons achieved in fullerene derivatives and can be regarded as unprecedented bridged binuclear carbon sandwiches. Although the construction does not follow the eight electron counting rule, these structures are true energy minima on the potential energy surfaces because the remainder electrons on the polar rings are delocalized throughout the whole cage framework to obtain further stabilization. Interestingly, the 2(N+1)2 rule is still valid for predicting the spherical aromaticities of these non-classical endofullerenes. These new carbon clusters would have less reactivity than fullerene C20 and should be viable synthetic candidates. Extension of our constructing strategy would lead to the discovery of many new non-classical compounds and nanostructures.