Heavy atom tunneling: When molecules disobey the reactivity textbook


Sebastian Kozuch


Ben-Gurion University of the Negev, Israel


Donostia International Physics Center (Hybrid Seminar)


Bo Chen

Quantum mechanical tunneling of “heavy” atoms1 (i.e. heavier than hydrogen) is extremely atypical, but also exceptionally rich chemistry, especially at cryogenic conditions. The only requirements are a rather low activation energy and, much more important, a narrow barrier. From all the cryptic effects can appear in such regime, we will consider:
I. Quantum tunneling instability,2 where supposedly stable hypothetical molecules can swiftly decompose, even when trying to keep them close to the absolute zero.
II. Isotope controlled selectivity,3 where the product of a reaction can be dictated by the presence of deuterium.
III. Reassessment of activation energies, where your ultra-accurate computed barriers might still be missing an important component.4
IV. Degenerate rearrangements, where the molecules just cannot stay put.  bond shifting,5 fluorine tunneling,6 transition metal complexes,7 heavy atom tunneling might appear just around the corner.
V. Tunneling inside Diamonds

1 W. T. Borden, Reactions that involve tunneling by carbon and the role that calculations have played in their study, WIREs Comput. Mol. Sci., 2016, 6, 20–46.
2 A. Frenklach, H. Amlani and S. Kozuch, Quantum Tunneling Instability in Pericyclic Reactions, J. Am. Chem. Soc., 2024, 146, 11823–11834.
3 A. Nandi, D. Gerbig, P. R. Schreiner, W. T. Borden and S. Kozuch, Isotope-Controlled Selectivity by Quantum Tunneling: Hydrogen Migration versus Ring Expansion in Cyclopropylmethylcarbenes, J. Am. Chem. Soc., 2017, 139, 9097–9099.
4 S. Kozuch, T. Schleif and A. Karton, Quantum mechanical tunnelling: the missing term to achieve sub-kJ mol−1 barrier heights, Phys. Chem. Chem. Phys., 2021, 23, 10888–10898.
5 A. Nandi, E. Solel and S. Kozuch, Carbon Tunneling in the Automerization of Cyclo[18]carbon, Chem. Eur. J., 2020, 26, 625–628.
6 I. Sedgi and S. Kozuch, Heavy atom tunnelling on XeF6 pseudorotation, Phys. Chem. Chem. Phys., 2020, 22, 17725–17730.
7 I. Sedgi and S. Kozuch, Heavy-atom tunnelling in Cu(II)N6 complexes: theoretical predictions and experimental manifestation, Chem. Sci., 2020, 11, 2828–2833.
8 I. Sedgi and S. Kozuch, A Playground for Heavy Atom Tunnelling: Neutral Substitutional Defect Rearrangement from Diamondoids to Diamonds, Chem. Eur. J., 2023, 29, e202300673.

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