Tunnelling above Room Temperature detected

It is one of the axioms of chemical kinetics that the height of the barrier between reactants and products determines the rate of the reaction. Another axiom is that selectivity in chemical reactions can be obtained, when a process has two possible barriers to traverse, if the two barriers are not of equal height. Rates can then be enhanced or selectivities decreased by increasing the temperature, which will negate the effect of the barrier(s) to some extent. This is expressed through e.g. the Arrhenius equation or the Eyring equation.

Tunneling. not over the barrier image

However, for reactions involving the motion of light atoms such as hydrogen, these rates can be significantly enhanced by so-called tunnelling through the barrier, a purely quantum mechanical effect. Hydrogen tunnelling is postulated to play a crucial room at very low temperatures in e.g. astrochemistry and at room temperature in e.g. biochemistry or catalysis. However, direct spectroscopy evidence (as opposed to indirect kinetic isotope effect measurements) is thus far limited to low temperatures, where classical 'over-the-barrier' reactivity is limited.

In a recent publication in the Journal of the American Chemical Society Dr Anthony Meijer and collaborators from the University of Cologne and Nijmegen have found direct spectroscopy evidence for quantum tunnelling in the isomerization of hydroxycarbenes to aldehydes using a combination of IRMPD spectroscopy coupled with DFT calculations. In the experiments a precursor molecule is dissociated to form either a carbene or an aldehyde. Because both the precursor as well as the carbene/aldehyde are charged, they can be kept isolated inside a mass spectrometer. Heating of the product carbene/aldehyde via IR photons then leads to dissociation and detection of the fragments via mass spectrometry. Enhanced efficiency of heating corresponds exactly to those frequencies where the molecules absorb IR photons effectively. In this particular case, the life-time of the carbene was followed, showing the decay into the aldehyde. By isotopic substitution of the tunnelling hydrogen, definite proof of tunnelling was obtained. These experiments were enhanced by calculations done in Sheffield, which proved that classical 'over-the-barrier' reactivity was very unlikely in this system and which proved the assignment of the experimental peaks, providing definite proof of the initial co-existence of carbene and aldehyde in the experimental setup.

Further Information.

The full article by Mathias Schäfer, Katrin Peckelsen, Mathias Paul, Jonathan Martens, Jos Oomens, Giel Berden, Albrecht Berkessel, and Anthony J. H. M. Meijer can be viewed here: Hydrogen Tunneling Above Room Temperature Evidenced by Infrared Ion Spectroscopy