Renner–Teller effect

https://doi.org/10.1351/goldbook.R05290
Molecular distortion in linear molecular species with degenerate electronic states (e.g. belonging to the C∞v group). The Renner–Teller effect is a particular case of the Jahn–Teller effect. From the point of view of the potential energy surfaces the Renner–Teller effect generates a surface touching (see scheme below).
Notes:
  1. The Renner–Teller effect arises from splittings in the vibrational levels of molecular entities due to even terms in the vibronic perturbation expansion. This is generally a minor effect for non-linear molecular entities compared to the @J03361@, which is due to the odd terms.
  2. For linear molecular entities it is the only possible vibronic effect characteristic of degenerate electronic states.
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Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 414 (https://doi.org/10.1351/pac200779030293)