Electronic excited states and stabilities of fullerenes: Isomers of C ₇₈ and Mg@C ₇₂

There is one factor in relative stabilities of isomeric fullerenes that has rarely been studied so far - the contribution of excited electronic states. The contribution is dearly quantified 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 article the problem is studied on two isomeric systems. One is the set of five isolated pentagon rule isomers of C ₇₈, relatively well known from experiments. The other is a model set of four isomers of Mg@C ₇₂ (not isolated yet). The electronic excitation energies are computed by the limited configuration interaction (CI) method. In the case of the C ₇₈ isomeric set, the ZINDO semiempirical method is used. Isomers of Mg@C ₇₂ are evaluated by means of the single-excitation CI or Q-singles (CIS) in the standard LanL2MB and LanL2DZ basis sets - CIS/LanL2MB and CIS/LanL2DZ. 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.