Quantum Mechanical Tunneling of Hydrogen Atoms in Some Simple Chemical Systems

Quantum Mechanical Tunneling of Hydrogen Atoms in Some Simple Chemical Systems 

Not only is it difficult to prove that a given reaction proceeds via tunneling, it is even difficult to define the term tunneling unambiguously. The hydrogen atoms themselves, one presumes, are unaware that they are tunneling from their vantage point the barrier is, like beauty, only present in the eye of the observer. Moreover, not all observers will see the barrier but only those that have not yet overcome their classical prejudices. 

In 1933 Bell [1] predicted that, due to quantum mechanical effects, the rate of transfer of a hydrogen atom (H-atom) or proton would become temperature independent at low temperatures. Since that time, kineticists have embraced the concept of quantum mechanical tunneling (QMT) so enthusiastically that it is frequently invoked on the flimsiest of experimental evidence, often using data obtained at, or above, room temperature. At such elevated temperatures, conclusive evidence that the rate of an H-atom or proton transfer is enhanced above that due to over the top of the barrier thermal activation, and can only be explained by there being a significant contribution from QMT, is rare. Significant has been italicized in the foregoing sentence because QMT will always make some contribution to the rate of such transfers. The QMT contribution to the transfer rate becomes more obvious at low temperatures. For this reason, the unequivocal identification of QMT in simple chemical systems requires that their rates of reaction be measured at low temperatures. 

In this chapter, a few simple unimolecular and bimolecular reactions will be described in which the rates of H-atom motion were measured down to very low temperatures. These kinetic measurements provide unequivocal evidence that QMT dominates the reaction rates over a wide range of temperatures. There are two common themes. First, all the experimental data were generated in my own 

Hydrogen-Transfer Reactions. Edited by J. T. Hynes, J. P. Klinman, H. H. Limbach, and R. L Schowen Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-30777-7 

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