Excited electronic states and relative stabilities of C₈₀ isomers

Very high temperatures of fullerene synthesis allow for a significant population of excited electronic states and thus for non-negligible electronic partition functions. The issue is studied on the C₈₀ isomeric system, as relatively low-lying electronic excited states are known in this case. The set consists of seven isolated pentagon-rule (IPR) cages. Computations suggest a species of D_5d symmetry as the lowest-energy structure in the set, although in observations a cage of D₂ symmetry is more populated. The electronic partition functions are evaluated by means of the ZINDO method. The computed effects of the electronic excited states are still larger than in the previously tested examples. It is shown, however, that for the special conditions of the fullerene synthesis/isolation, the electronic partition function based on the singlet excited states only should better reflect the experimental population findings. The study is complemented with the MP2=FC/6-31G* evaluation of the separation energy between the D _5d and D₂ isomers that places the D₂ species ∼2.8 kcal/mol below the D_5d cage, i.e., the reversed order than in several previous calculations.