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

The UV spectrum of linear C-6 has been observed, and correlates well with previous theoretical results. However, for the D-3h structure of C-6, 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 D-3h and D-6h structures of C-6, were calculated by MR-CI methods. For the D-3h singlets, with an X(1)A(1)' ground state, intense transitions are predicted to 1(1)E' at 2.96 eV, and to 5(1)E' at 6.9 eV. For the D-6h singlets, with an X(1)A(1g) ground state, the most intense transition is predicted for 1(1)E', at 4.8 eV. Comparison with the pi-pi* spectrum of benzene has been made. The lowest triplet states lie 2.16 eV above X(1)A(1)' for D-3h, and 1.90 eV above X(1)A(1g) for D-6h symmetry. (C) 2001 Elsevier Science B.V. All rights reserved.