C78 IPR fullerenes:: Computed B3LYP/6-31G*//HF/3-21G temperature-dependent relative concentrations

The five isolated-pentagon-rule (IPR) satisfying isomers Of C-78 labeled 1-5, or according to symmetry as D-3, C-2v, C-2v', D-3h, and D-3h', are computed. The cage geometries are optimized at the ab initio HF level with the standard 3-21G basis set (HF/3-21G). The separation energetics is then computed using the B3LYP density-functional treatment in the standard G-31G* basis set (B3LYP/6-31G*//HF//3-21G). Harmonic vibrational frequencies arc calculated by the SAM1 semiempirical method. The computed energies, structural and vibrational data arc, employed in the construction of isomeric partition functions and evaluation of the relative Gibbs free energies. The results are converted into relative concentrations for a wide temperature interval. The C-2v', structure is the most populated throughout while the D-3h, species is negligible at all temperatures. The agreement between theory and experiment, is reasonable, though some aspects are still to be clarified.