The additional electron in the C^ anion occupies the lower a2u state. The tlg orbital must also split under this distortion, tlg elg + alg. Three transitions (one of these being weak) are identified with the optical absorption bands in trace B of Fig. 13.20. The transitions a2u —>■ elg in Fig. 13.22(B) are allowed and are identified with the peak at 1078 nm in trace B of Fig. 13.20. The weaker transitions a2u -*■ a2g are forbidden and are identified with a vi-bronic manifold between 800 and 1000 nm. These groups of transitions are labeled and correspond to the peaks identified in the simulated spectrum above the level diagram.
In column C of Fig. 13.22, the level diagram proposed to explain spectrum C in Fig. 13.20 for C^ is displayed. This anion has been reported by some authors to have a triplet ground state (5 = 1) from EPR studies (see §16.2.2) [13.140]. Thus, the doubly degenerate elu state is placed below the singly degenerate a2u level. Two dipole-allowed transitions are expected: elu -> elg (b), which is identified with the absorption band at 952 nm; and e}u -> a2g (a), which is identified with the absorption band at 830 nm.
The spectra for the Cf)0 and C^ anions are more complex, and the assignments are more speculative. Turning to the trianion first, the proposed level scheme is shown in column A of Fig. 13.23. Based on observed EPR spectra, C^ has been reported to resemble a spin 1/2 system (see §16.2.2) [13.141]. It is assumed that a Jahn-Teller distortion occurs for the Cgô anion to remove the ground state degeneracy and lower the system energy. A C2v
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