Jahn-Teller weakening

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. 

The cyclobutane radical cation was calculated by several groups, who evaluated Jahn-Teller distorted structures potentially arising upon ionization of cyclobutane [79, 83-87]. Four distorted local minima were considered (Figure 13). A rectangular structure with two weakened C-C bonds (type A) and a rhomboidal structure with four weakened C-C bonds (type B) result from first order Jahn-Teller distortion. Two further structures, a trapezoidal one (one weakened C-C bond type C), and an irregular structure shaped like a kite (two lengthened C-C bonds type D) can be envisaged as a consequence of second-order Jahn-Teller distortion [86]. A detailed recent study also considered puckered equivalents of the planar structure types, A-D... 

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