Quantum Tunneling Dynamics in Chemical Reactions

Quantum tunneling refers to the ability of atoms or electrons to traverse energy barriers that would be insurmountable under classical mechanics. In chemical reactions, tunneling can accelerate bond rearrangements, proton or heavy-atom transfers and conformational isomerisations even at cryogenic temperatures. Beyond the familiar picture of activation energy, tunneling dynamics hinge on both barrier height and barrier width as defined on the molecular potential energy surface. Reactions that proceed via tunneling often exhibit temperature-independent rate constants below a threshold temperature and unusually large kinetic isotope effects when hydrogen is substituted by deuterium. Tunneling phenomena are pivotal in enzymatic catalysis, where protein vibrations may couple to the reactive coordinate, in astrochemical processes occurring in interstellar ices and in the design of molecular machines and quantum devices. Recent advances in ultrafast spectroscopy, low-temperature matrix isolation and atomically resolved microscopy have provided direct observation of tunneling pathways, while improvements in theoretical methods—such as variational transition state theory with multidimensional tunneling corrections—have yielded quantitative rate predictions. A deeper understanding of tunneling dynamics is now informing the rational design of catalysts and materials with tailored quantum behaviours.

Research from Nature Portfolio

Recent computational studies have employed density functional theory combined with variational transition state theory and Wigner tunneling corrections to elucidate the kinetics of tautomerisation and rotamerisation in heterocyclic systems. These investigations demonstrate that quantum tunneling can dominate the interconversion rates of molecular conformers even at ambient temperatures, leading to solvent-dependent reversibility and unexpected conformer stability orders. The calculations reveal that inclusion of multidimensional tunneling corrections is essential for accurate rate constants and that subtle changes in solvation can reorder relative energies of reactive conformers. Such findings underscore the need to account for quantum effects in mechanistic models of chemical reactivity and in the interpretation of spectroscopic data.

Quantum Tunneling Dynamics in Chemical Reactions publication trend

The graph below shows the total number of articles in quantum tunneling dynamics in chemical reactions across all publications each year (not limited to Nature Index journals).

Technical terms

Quantum tunneling: The phenomenon by which particles traverse energy barriers higher than their classical kinetic energy, enabling reactions or rearrangements at low temperatures.

Potential energy barrier: The region of elevated energy separating reactant and product minima on a molecule’s potential energy surface, characterised by its height and width.

Intrinsic barrier width: The spatial extent of the potential energy barrier independent of thermodynamic driving force, critically influencing tunneling probabilities.

Variational transition state theory: A theoretical framework for calculating chemical reaction rates that optimises the dividing surface and incorporates multidimensional tunneling corrections.

Kinetic isotope effect: The change in reaction rate observed when an atom (commonly hydrogen) is substituted by a heavier isotope, often magnified by tunneling contributions.

References

1. The Intrinsic Barrier Width and Its Role in Chemical Reactivity. ACS Central Science (2023).
2. Simultaneous Tunneling Control in Conformer-Specific Reactions. Journal of the American Chemical Society (2022).
3. A density functional theory study of the molecular structure, reactivity, and spectroscopic properties of 2-(2-mercaptophenyl)-1-azaazulene tautomers and rotamers. Scientific Reports (2023).
4. Nuclear quantum tunnelling in enzymatic reactions – an enzymologist's perspective. Physical Chemistry Chemical Physics (2015).
5. Real-Space Observation of Quantum Tunneling by a Carbon Atom: Flipping Reaction of Formaldehyde on Cu(110). The Journal of Physical Chemistry Letters (2019).