Term: light-atom anomaly
https://doi.org/10.1351/goldbook.L03523

Definition:
A dynamical effect that arises for a process wL03523.png  hen the species A is light (e.g. a hydrogen atom) compared to B and C. The vibrational excitation of the product A–B is low, since the light atom A approaches to within the bonding distance of BC before the C atom retreats. The energy of reaction is therefore released as repulsion between A–B and C, with the result that there is translational excitation of the products.

Image: Not defined (https://goldbook.iupac.org/img/inline/L03523.png) 

Source: PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 171 (https://doi.org/10.1351/pac199668010149)

Citation: 'light-atom anomaly' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.L03523

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