Hydrogen tunnelling experimental evidence

In Chapter 8, Stavola and Pearton discuss the local vibrational modes of complexes in Si that contain hydrogen or deuterium. They also show how one can use applied stress and polarized light to determine the symmetry of the defects. In the case of the B-H complex, the bond-center location of H is confirmed by vibrational and other measurements, although there are some remaining questions on the stress dependence of the Raman spectrum. The motion of H in different acceptor-H complexes is discussed for the Be-H complex, the H can tunnel between bond-center sites, while for B-H the H must overcome a 0.2 eV barrier to move between equivalent sites about the B. In the case of the H-donor complexes, instead of bonding directly to the donor, H is in the antibonding site beyond the Si atom nearest to the donor. The main experimental evidence for this is that nearly the same vibrational frequency is obtained for the different donor atoms. There is also a discussion of the vibrational modes of H tied to crystal defects such as those introduced by implantation. The relationship of the experimental results to recent theoretical studies is discussed throughout. 

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. 

Multiple-position KIEs, when combined with computational modeling, can provide enough information to successfully model a reaction coordinate and demonstrate hydrogen tunneling, even when the primary kn/feo ratio is not enormous. More complete experimental evidence for tunneling can be obtained by demonstrating a breakdown in the exponential relationships (e.g. kn/feT versus ko/kx) or by variable-temperature KIE measurements. 

S. J., Klinman, J. P. (1996) Experimental evidence for extensive tunneling of hydrogen in the lipoxygenase reaction - implications for enzyme catalysis, J. Am. Chem. 

Volume 2 concludes in Part VII with contributions on the variational transition state theory approach to hydrogen transfer in various contexts (Truhlar and Garrett, Ch. 27), on experimental evidence of hydrogen atom tunneling in simple systems (Ingold, Ch. 28), and finally on a theoretical perspective for multiple hydrogen transfers (Smedarchina, Siebrand and Fernandez-Ramos, Ch. 29). 

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