The ground state wavefunction of the neutral icosahedral C60 molecule belongs to the total symmetric one-dimensional Ag representation. However, the degeneracy of the HOMO is five-fold while the LUMO is triply degenerate. This means that the lowest excited and ionized many-electron states will also be degenerate, and thus they are subjected to Jahn—Teller distortions. In this work we used a simple model Hamiltonian to study the extent and energy of the distortions arising due to excitations. For the π-electrons we used a Pariser—Parr—Pople type wavefunction augmented by an empirical potential to describe the σ cores. The π-electron part of the Hamiltonian depends on the bond lengths which can be optimized by means of simple gradient techniques. The excited states are described by the Tamm — Dancoff approximation (all single CI). In order to locate the distorted states on the energy hypersurface, the degenerate excited states were reduced according to those subgroups of the Ih group that contain one-dimensional (that is, Jahn-Teller inactive) irreducible representations in an excitation subspace. Distorted structures of D2h, D3dD5d and Th symmetries were determined. The extent of the distortions is small, the largest change in bond lengths being 0.02 Å. The Jahn Teller distortion energies were found to be typically 1–3kcalmor−1. Singlet and triplet excited state spectra were computed using the CNDO/S-CI method. Comparison of the calculated spectra with experimental results shows evidence of Jahn-Teller distortions.
ASJC Scopus subject areas
- Condensed Matter Physics
- Physical and Theoretical Chemistry