Molecular modelling Jahn-Teller distortions

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. 

From the data on divalent and tetravalent salts, a consistent picture emerges which is in perfect agreement with the Mott-Jahn-Teller insulator model of Fabrizio and Tosatti [16]. The Jahn-Teller distortion, even if dynamic, can be unambigously detected from vibrational and low-energy electronic spectra and proves that the molecular JT effect causes the nonmagnetic insulating behavior in these materials. Systems with smaller cations are on the metallic side of the UAV diagram it would be of interest to study these systems by vibrational spectroscopy as well. 

An interesting case of a Jahn-Teller distorted hexacoordinate copper(II) complex is shown in Figure 12.3. Here, the elongation along the O—Cu—O axis leads to a loss of delocalization within the hfacac ligand skeleton. Clearly, the simple electronically doped molecular mechanics model will not be able to predict this additional distortion within the organic part of the molecule. 

Various other interactions have been considered as the driving force for spin-state transitions such as the Jahn-Teller coupling between the d electrons and a local distortion [73], the coupling between the metal ion and an intramolecular distortion [74, 75, 76] or the coupling between the d electrons and the lattice strain [77, 78]. At present, based on the available experimental evidence, the contribution of these interactions cannot be definitely assessed. Moreover, all these models are mathematically rather ambitious and do not show the intuitively simple structure inherent in the effect of a variation of molecular volume considered here. Their discussion has to be deferred to a more specialized study. 

Kambara presented a ligand field theoretical model for SCO in transition metal compounds which is based on the Jahn-Teller coupling between the d-electrons and local distortion as the driving force for a spin transition [193]. The author applied this model also to interpret the effect of pressure on the ST behaviour in systems with gradual and abrupt transitions [194]. By considering the local molecular distortions dynamically this model turned out to be suited to account for cooperative interactions during the spin transition [195]. 

We discuss here two examples of vibronic effects in polynuclear highly symmetrical transition metal clusters. The existence of degenerate and quasi-degenerate molecular orbitals in their energy spectra results in the Jahn-Teller effect or in the vibronic mixing of different electronic states. We show that both quantum-chemical methods and model approaches can provide valuable information about these vibronic effects. In the case of the hexanuclear rhenium tri-anion, the Jahn-Teller effect is responsible for the experimentally observed tetragonal distortion of the cluster. The vibronic model of mixed-valence compounds allows to explain the nature of a transient in the photo-catalytic reaction of the decatungstate cluster. 

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