First ab Initio and Density Functional Study on the Structure, Bowl-to-Bowl Inversion Barrier, and Vibrational Spectra of the Elusive C₃v-Symmetric Buckybowl:  Sumanene, C₂₁H₁₂

The synthetically elusive C3v symmetric sumanene (C21H12), a key structural motif of C60, was subjected to a detailed computational study, exploring the structure, bowl-to-bowl inversion dynamics, vibrational spectra, and some other physicochemical properties. Hartree−Fock (HF), pure (BLYP, BP86, and BPW91), and hybrid density functional (B3LYP, B3P86, and B3PW91) calculations were done with an array of basis sets (STO-3G, 3-21G, 6-31G*, 6-31G**, 6-311G*, 6-311G**, 6-311+G*, 6-311++G*, cc-pVDZ, and cc-pVTZ). The effect of a basis set higher than double-ζ quality and the inclusion of dynamic correlation on the geometry and bowl-to-bowl inversion barrier was insignificant. The B3LYP or HF method with the cc-pVDZ or 6-311G* basis set gave satisfactory results. The previously computed modified neglect of diatomic overlap (MNDO) value of 24.2 kcal/mol for the bowl-to-bowl inversion was found to be too high, and a revised value of 16.9 kcal/mol was obtained by the B3LYP/cc-pVTZ//B3LYP/cc-pVDZ method. Consequently, the computed results indicate that sumanene (2) is not locked in the bowl geometry and that a definitive bowl-to-bowl inversion should exist at room temperature. The highest level of theory used in the study (B3LYP/6-311G**) yields values of 1.14 Å, 2.45 D, and 98.8° for the bowl depth, dipole moment, and π-orbital axis vector angle at the hub carbon for sumanene, respectively. Interesting temperature dependency of inversion dynamics is predicted near room temperature.