C-78 IPR fullerenes: Computed B3LYP/6-31G*//HF/3-21G temperature-dependentrelative 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 compu ted. The cage geometries are optimized at the ab initio HF level with the s tandard 3-21G basis set (HF/3-21G). The separation energetics is then compu ted using the B3LYP density-functional treatment in the standard G-31G* bas is set (B3LYP/6-31G*//HF//3-21G). Harmonic vibrational frequencies arc calc ulated by the SAM1 semiempirical method. The computed energies, structural and vibrational data arc, employed in the construction of isomeric partitio n 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, spe cies is negligible at all temperatures. The agreement between theory and ex periment, is reasonable, though some aspects are still to be clarified.