Observation of hydrogen tunneling in an isolated system at room temperature
Tunneling is a quantum-mechanical phenomenon where a particle proceeds from one side of a potential barrier to the other even though it possesses an energy that is lower than the barrier height, a process that is classically strictly forbidden. This phenomenon is postulated to play a crucial role in low temperature chemistry, for instance in astrochemistry, where thermal reactivity is inherently low. Various electron transfer processes in biochemistry and catalysis at room temperature have also been attributed to tunneling. A team of researchers from the FELIX Laboratory in Nijmegen and the universities of Cologne (D) and Sheffield (UK) have now uncovered a particularly clear example of room-temperature tunneling of a hydrogen atom in an isolated system. This study is recently published in the Journal of American Chemical Society.
Reaction kinetics are commonly described by theories using the reaction barrier as a central parameter. However, for reactions involving the motion of light particles, a significant enhancement of the reaction rate may be observed due to a quantum-mechanical effect known as “tunneling”. The quantum-mechanical wavefunction describing a particle can extend substantially into the barrier, the classically forbidden regions of a potential energy surface. The lighter the particle, the further the penetration into the barrier, so that wavefunctions describing light particles can have non-negligible amplitude on the other side of the barrier. In other words, there is a non-negligible probability to find the particle on the other side; in terms of a chemical reaction, the reactants can proceed toward the products even if the energy available in the system is lower than the barrier height.
The researchers prepared a molecular ion possessing a hydroxycarbene moiety and mass-isolated it in an ion trap mass spectrometer. The molecular structure of the ion is established by its in-situ recorded IR spectrum facilitated by use of the tunable IR radiation from the free electron laser.Varying the storage time of the ions in the trap, the IR spectrum recorded after different delay times is observed to change, indicating the spontaneous rearrangement of the carbene ion to its aldehyde isomer.
Figure: Isomerisation of the hydroxycarbene ion on the left does not happen over the barrier, but instead proceeds via quantum-mechanical tunneling to form the aldehyde on the right.
Quantum-chemical computations suggest that the barrier for this reaction is well above the energy available in the system at room temperature. Nonetheless, the reaction involving the transfer of a proton proceeds at a rate of approximately 0.1 sec-1, indicating the involvement of hydrogen tunneling. Furthermore, repeating the experiment with the hydroxyl proton replaced by a deuteron, no rearrangement is observed even at long storage times, which is interpreted as being a consequence of the strong mass dependency of the tunnel rate.
Reference:
Publication: Hydrogen Tunneling Above Room Temperature Evidenced by Infrared Ion Spectroscopy, M. Schäfer, K. Peckelsen, M. Paul, J. Martens, J. Oomens, G. Berden, A. Berkessel, A.J. H. M. Meijer, J. Am. Chem. Soc.
Further information:
Prof. J. Oomens, Radboud University, j.oomens@science.ru.nl, 024-3653950