Jahn-Teller excited states

Ab initio molecular orbital calculations for the model systems RCN3S2 (R = H, NH2) show that these dithiatriazines are predicted to be ground state singlets with low-lying triplet excited states (Section 4.4). The singlet state is stabilized by a Jahn-Teller distortion from C2v to Cj symmetry. In this context the observed dimerization of these antiaromatic (eight r-electron) systems is readily understood. 

The splitting of the asymmetric peak is ascribed to a Jahn-Teller splitting of the excited state which latter involves the open e configuration e ... 

Recently, a symmetry rule for predicting stable molecular shapes has been developed by Pearson Salem and Bartell" . This rule is based on the second-order, or pseudo, Jahn-Teller effect and follows from the earlier work by Bader . According to the symmetry rule, the symmetries of the ground state and the lowest excited state determine which kind of nuclear motion occurs most easily in the ground state of a molecule. Pearson has shown that this approximation is justified in a large variety of inorganic and small organic molecules. 

Bojan, V.R., Fernandez, E.J., Laguna, A., L6pez-de-Luzuriaga, J.M., Monge, M., Olmos, M.E. and Silvestru, C. (2005) Phosphorescent excited state of [Au2 (Ph2Sb)20 3] + Jahn-Teller distortion at only one gold(I) center. Journal of the American Chemical Society, 127, 11564-11565. 

How complicated the nature of these excited states can be, has been shown convincingly by Van der Waals et al., who studied these states by resonance techniques. A strong Jahn-Teller distortion appears to be present [28-30]. 

Csv chromophores, may perhaps be attributed to a Jahn-Teller effect in the excited E state (13). 

Fig. 4. Cr(CO)s excited state relaxation dynamics comparison of semi-classical trajectory surface hopping (left), and MCTDH wave packet dynamics (right). Trajectory shows molecule passing through TBP Jahn-Teller geometry within 130 fs, then oscillating in SP potential well afterward. Wave packet dynamics plotted for the Si and S0 adiabatic states in the space the symmetric and asymmetric CCrC bending coordinates.

Fig. 7. Potential energy surfaces for ground and first excited state of Mo2(DXylF)2(02CCH3)2(n2-0)2 complex. The pseudo-Jahn-Teller effect results from a coupling between So and the excited 1( 6 ) state, causing the central Mo2(p2-0)2 motif to be rhomboidal (C2h), rather than square (D2h) [Adapted from Ref. (55) with permission].

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