Jahn-Teller static effect

Now, we examine the effect of vibronic interactions on the two adiabatic potential energy surfaces of nonlinear molecules that belong to a degenerate electronic state, so-called static Jahn-Teller effect. 

Ballhausen, C. and H. Johansen Molecular-orbital calculation of static Jahn-Teller effect in CuFg. Mol. Phys. 10, 183 (1966). 

It is interesting to note that the same group also found evidence for the formation of Jahn-Teller-stabilized spin-singlets (C q and C o) also in Na2C60 and Na2CsC60 [55]. This system is at low temperatures orientationally ordered and crystallizes in the Pa3 space group. This suggests that orientational order/dis-order of ions may be important for the occurrence of a static Jahn-Teller effect in these compounds. 

The choice of this example may seem unnecessarily complicated to start with. However, we have the choice between the mathematical complications of calculating interelectronic repulsion parameters and the physical complications that for instance d1- and d9-systems such as Ti(H20) +3 and Cu(H20) ++ are Jahn-Tc/fer-distorted. Whether one has a static Jahn-Teller effect where the stereochemical configuration of the ground-state is distorted, or one has a dynamic Jahn-Teller effect where the potential surfaces of the excited and groundstates are covered by un-symmetric vibrations, is a subtle question we do not need to treat here. However, it is an empirical fact that these Jahn-TeZ/er-unstable molecules do not have the simple spectra expected from higher symmetries such as the octahedral Oh. 

The static Jahn-Teller effect can be observed only in the presence of an external influence. Bersuker [76, 77] stresses this point as the opposite statement is found often in the literature. According to the statement criticized, the effect is not to be expected in systems where low-symmetry perturbations remove electronic degeneracy. However, it is exactly the low-symmetry perturbations that make the Jahn-Teller distortions static and thus observable. Such a low-symmetry perturbation can be the substitution of one ligand by another. In this case one of the previously equivalent minimum-energy structures, or a new one, will become energetically more favored than the others. 

Lattice vibrations tend to destroy the correlation among Jahn-Teller centers. Thus, with increasing temperature, these centers may become independent of each other at a certain point, and their static Jahn-Teller effects convert to dynamic ones. At this point the crystal as a whole becomes more symmetric. This temperature-dependent static -O- dynamic transition is called a Jahn-Teller phase transition. Below the temperature of the phase transition, the cooperative Jahn-Teller effect governs the situation providing static distortion the overall structure of the crystal is of a lower symmetry. Above this temperature, the cooperation breaks down, the Jahn-Teller distortion becomes dynamic and the crystal itself becomes more symmetric. 

Laser-vaporized Nb and Ta were condensed in D2 or codeposited with H2 in rare gas matrices at low temperature.535 EPR spectra detected MH4 as molecules of D2d point symmetry in 2Bi ground states imposed by a static Jahn-Teller effect. IR spectra indicated an H—Nb—H bond angle of 130°. 

Stereochemical consequences of the Jahn-Teller theorem are straightforward the higher symmetric polyhedra in electronic degenerate states should distort in order for their electronic ground state to become non-degenerate this is the static Jahn-Teller effect. Some possible distortions are exemplified by ... 

---