Spectroscopy dynamic Jahn-Teller effect

Spin-crossover phase transition of a manganese(IU) complex [Mn(taa)] was studied by variable-temperature laser Raman spectroscopy and it was found that the vibrational contribution in the transition entropy is not dominant in contrast to the cases of ordinary iron spin-crossover systems. The discovery of a dynamic disorder in the HS phase by means of dielectric measurements provided an alternative entropy source to explain the thermally induced spin-crossover transition. This dynamic disorder was attributed to the reorienting distortion dipoles accompanying the E e Jahn-Teller effect in HS manganese(III) ions. 

The timescale of infrared spectroscopy is such that it is capable of detecting dynamic distortions. This method is sensitive to the local structure, so that it detects the distortions of the single molecules and not their average. The MIR spectra of the A4C60 compounds at high temperatures show a twofold split Ti (4) mode (Fig. 7) corresponding to either a T>j,d or a Dsd distortion (see Table 1.) [4,69]. The NIR spectra contain two peaks (Fig. 7), which also correspond to D d or Dsd structures (see Fig. 8) [4]. These are the distortions favored by the molecular Jahn-Teller effect. It has been shown that at high temperatures these distortions are dynamic [4]. 

We have studied the Jahn-Teller effect in this phase by MIR spectroscopy [31, 70]. The Ti (4) mode shows a twofold splitting and modes that are silent in Ceo appear, indicating a D3 or T>sd distortion of the molecule (Fig. 10). As the C o ions are rotating in this phase [73,74], the distortion cannot be caused by the crystal field but must be due to the molecular Jahn-Teller effect. As there is no crystal field to lock the Cgo into a single potential well, the distortion is dynamic, with the rate of pseudorotation smaller than that of the infrared measurement. 

Bersuker, I.B. The Jahn-Teller Effect. Cambridge University Press, Cambridge (2006) Domcke, W., Yarkony, D.R., Koppel, H. Conical Intersections, Electronic Structure, Dynamics and Spectroscopy. Advanced Series in Physical Chemistry, vol. 15. World Scientific, Singapore (2004)... 

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. 

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