# Are there any non-Jahn-Teller type distortions?

My question is essentially, if ANY gradual type of deviation from a higher possible symmetric arrangement of the nuclei in a molecule is covered by the Jahn-Teller model.

More specifically I have in mind some lets say "ionic complexes" between a very small cation and some larger (possibly weakly coordinating) anions.

Very specifically, I remember I have seen for example a calculated equilibrium structure of a beryllium bis-2-fluorobenzoate $\ce{(Be[o-C6H4FCOO]2}$, isomer with $\ce{F\bond{...}Be-O}$ coordination), which showed a distinct deviation from $\ce{C2}$ symmetry. The only type of JT-distortion this would qualify for is 2nd order one. But then since its largely an ionic species I suppose the HOMO-LUMO gap should be so large that 2nd order JT is very unlikely (I am aware that $\ce{C2}$ is not Jahn-Teller active but through vibronic coupling of close-lying $\ce{^2A}$ and $\ce{^2B}$ also JT-like distortions can occure).

I imagine that if the cation is very small so that it "dangles" in the gap between the anions it might be energetically more favourable if the cation decides to stick on one than to balance in the centre. Just a naive picture. But would such a thing - if it exists - be subsumed under JT as well?

So, is distortion possible for ionic aggregates and would such a thing also be described by JT?

• Are sterically driven distortions in scope? Nov 29 '17 at 1:47
• @OscarLanzi: Yes. Dec 25 '17 at 14:58