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As we all know $\ce{H2O}$ is bent ($C_\mathrm{2v}$) in its ground state equilibrium structure, rather than linear ($D_\mathrm{\infty h}$), which can be readily understood e.g. from the MO diagram of bent versus linear case:

H2O at linear geometry

H2O at bent geometry

The question, that puzzles me here, is if that apparent case of "symmetry breaking" can be described by a Jahn-Teller or Renner-Teller effect? Jahn-Teller seemingly is ruled out since the "high symmetric" from is linear, which is excluded from the Jahn-Teller theorem while the Renner-Teller effect is apparently a subtle ro-vibronic effect.

Since the water ground state is a closed shell singlet (even in the linear case), my take on it would be, that it must be a second-order JT-effect of coupling between ground and excited states by virtue of the bending distortion. But I am not sure since I never heard or read about water as a second-order JT-example.

Can anyone comment on that?

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    $\begingroup$ Check out Albright et al. Orbital Interactions in Chemistry 2ed pp 131-9 $\endgroup$ Commented Jan 23, 2019 at 12:43
  • $\begingroup$ @orthocresol: So it means: Perfectly normal second order JT, if I understand correctly? $\endgroup$ Commented Jan 23, 2019 at 13:40
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    $\begingroup$ I think so, but will refrain from giving a more certain answer, I only briefly flicked through. $\endgroup$ Commented Jan 23, 2019 at 14:03
  • $\begingroup$ I don't think so. If I remember correctly, the ground state of linear water is a sigma state, so that excludes it from any Jahn-Teller or Renner-Teller effects. Similar phenomena, (such as 2nd order JT or pseudo-JT) require the presence of low-lying excited states, which is not the case in linear water. The first excited state is several eV higher in energy, not enough for a significant vibronic effect. $\endgroup$
    – johnymm
    Commented Aug 18, 2022 at 23:55
  • $\begingroup$ I don't think so. Strictly speaking there is no kind of structural distortion between a higher and a lower symmetry structure at all that cannot be described by perturbation theory in chemistry. It may require high order PT like 3,4,5th and so on, however. In the end it may depend only on how you define the JT effect. $\endgroup$ Commented Aug 20, 2022 at 3:24

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