Journal
NATURE
Volume 612, Issue 7941, Pages 691-+Publisher
NATURE PORTFOLIO
DOI: 10.1038/s41586-022-05451-0
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Funding
- Alexander von Humboldt Foundation
- Deutsche Forschungsgemeinschaft [Sa 548/18-1]
- International Max Planck Research School for Elementary Processes in Physical Chemistry
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Quantum mechanical tunnelling describes the transmission of matter waves through a barrier with height larger than the energy of the wave. It becomes important when the de Broglie wavelength of the particle exceeds the barrier thickness. In condensed-phase chemistry, there are examples where increasing mass leads to increased tunnelling rates, showing nonmonotonic mass dependence.
Quantum mechanical tunnelling describes transmission of matter waves through a barrier with height larger than the energy of the wave(1). Tunnelling becomes important when the de Broglie wavelength of the particle exceeds the barrier thickness; because wavelength increases with decreasing mass, lighter particles tunnel more efficiently than heavier ones. However, there exist examples in condensed-phase chemistry where increasing mass leads to increased tunnelling rates(2). In contrast to the textbook approach, which considers transitions between continuum states, condensed-phase reactions involve transitions between bound states of reactants and products. Here this conceptual distinction is highlighted by experimental measurements of isotopologue-specific tunnelling rates for CO rotational isomerization at an NaCl surface(3,4), showing nonmonotonic mass dependence. A quantum rate theory of isomerization is developed wherein transitions between sub-barrier reactant and product states occur through interaction with the environment. Tunnelling is fastest for specific pairs of states (gateways), the quantum mechanical details of which lead to enhanced cross-barrier coupling; the energies of these gateways arise nonsystematically, giving an erratic mass dependence. Gateways also accelerate ground-state isomerization, acting as leaky holes through the reaction barrier. This simple model provides a way to account for tunnelling in condensed-phase chemistry, and indicates that heavy-atom tunnelling may be more important than typically assumed.
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