Theoretical studies on the electronic spectra of cyclic C6, in D3h and D6h symmetries Original Research Article

The UV spectrum of linear C6 has been observed, and correlates well with previous theoretical results. However, for the D3h structure of C6, predicted to be more stable, no UV spectrum is available. Using a 9s5p1d/5s3p1d basis set, vertical excitation energies and oscillator strengths of both singlet and triplet states, for D3h and D6h structures of C6, were calculated by MR-CI methods. For the D3h singlets, with an X 1A1′ ground state, intense transitions are predicted to 11E′ at 2.96 eV, and to 5 1E′ at 6.9 eV. For the D6h singlets, with an X 1A1g ground state, the most intense transition is predicted for 1 1E1u at 4.8 eV. Comparison with the π–π* spectrum of benzene has been made. The lowest triplet states lie 2.16 eV above X 1A′1 for D3h, and 1.90 eV above X 1A1g for D6h symmetry.