Enhancement of fullerene stabilities from excited electronic states

There is one factor in relative stabilities of isomeric fullerenes that has rarely been studied so far – contribution of excited electronic states. The contribution is clearly defined by the electronic partition function, supposing the related excitation energies can be evaluated. As temperatures in fullerene synthesis are high, the term should be taken into account. In this report the problem is studied on four isomeric systems. The first system is the set of five IPR (isolated pentagon rule) isomers of C78, relatively well known from experiment. The second system studied is a model set of four isomers of Mg@C72 (not yet isolated species). The third case consists of seven IPR isomers of C80. The last set is formed by six C74 cages. The electronic excitation energies are computed by limited configuration interaction (CI) approach, mostly with the ZINDO semiempirical method. Isomers of Mg@C72 are evaluated by means of the single-excitation CI or CI-Singles (CIS) in the standard LanL2DZ basis set. It is found that the electronic partition function can cause significant changes in the computed equilibrium relative concentrations of isomers at high temperatures. Metallofullerenes are more likely candidates for such enhanced effects.