Instability Jahn-Teller

Figure 7.11 Demonstration of first and second order Jahn-Teller instabilities in LnNijI C. The Jig orbital is partially occupied and can couple with the LUMO via a bending mode. Since the HOMO is Jt-nonbonding, this orbital picture can give rise to a dynamic Jahn-Teller...

It is not too surprising to find non-metallic d6 and metallic d7 and d9 representatives also among the marcasites1. However, marcasites form also with d2 and d4 cations, which are Jahn-Teller instable. Since the d4 compounds reveal no spin moment we expect a decrease of the symmetry which is sufficient to split the three de levels in such a way that two of them are stabilized while the third is markedly destabilized. In fact,... 

Connections between geometric and electronic structure the Peierls instability and its relationship with Jahn-Teller instability... 

This effect is called Peierls instability and the astute reader will recognize that it is directly related to first-order Jahn-Teller instability in molecular chemistry. For... 

In general, then, a band which is 1/3,1/4. .. l/p occupied is subject to Peierls instability with respect to a tri-, tetra-, / -merisation. It is important to remind you at this point that the possibility of a Jahn-Teller instability does not always mean the distortion occurs. The same applies for Peierls instability. There are several reasons. First of all, Peierls instability holds rigorously for ideal one-dimensional materials and reality is a more complicated three-dimensional space. Second, as in the case of molecules, when high-spin states are preferred, the distortion is not favored (see Problem 7 for an example). Third, even in the case of low-spin systems, the distortion may have additional consequences, e.g., weakening of spectator bonds. Coupled destabilizing effects can dampen or even prevent distortion. In the H2 case described above, there is no effect opposing the distortion, and dimerization is complete, i.e., fully independent H2 molecules result from our hypothetical chain. 

Before leaving this section, we need to tell you a very important point that beginners often forget. As in the case of Jahn-Teller instability, Peierls instability occurs for particular electron counts. For example, dimerization (a distortion leading to the doubling of the elementary unit cell parameter) is expected to occur only in the case of a half-filled band (or nearly half-filled if the material is not stoichiometric), i.e., Peierls instability depends on band population. 

Jahn-Teller Instability in Spin-Frustrated Metal Clusters... 

The Hamiltonian for the molecules including the Jahn-Teller instability is,... 

We had the help of the Bologna theoretical group with whom I had a long association. Palmieri, Zauli, Styles and I calculated that in spirononatetraene the magnitude of the Jahn-Teller instability is a torsional displacement of 2.5° and a well depth 110 cm 1 [149]. One would expect that the molecule would acquire chirality by induction when dissolved in one enantiomer of a chiral substance, in which one torsionally displaced form would be stabilized below the other. Efforts have been made to verify this. The spectra are complex and have not yet been analyzed in enough detail. 

Later Narayan [63] and Rabalais and Katrib [64] further examined the assignment of the low frequency PE bands as Turner et al. [57] they all came to the conclusion that the first two bands must be due to the components of the Jahn-Teller split g state of C2H6. They pointed out that the vibration causing Jahn-Teller instability must be one of the degenerate CH3 deformation vibrations, Vg (ej or Vj j ( g). Their frequencies in the neutral molecule are 1486 and 1460 cm", respectively. 

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