Deuterium atom tunnelling

Hydrogen and deuterium atom tunnelling has been reported to be common in prototropic organic reactions. The importance of tunnelling to fully understand chemical reactivity in organic chemical reactions has been established. ... 

Based on C-H versus C-D zero point vibrational differences, the authors estimated maximum classical kinetic isotope effects of 17, 53, and 260 for h/ d at -30, -100, and -150°C, respectively. In contrast, ratios of 80,1400, and 13,000 were measured experimentally at those temperatures. Based on the temperature dependence of the atom transfers, the difference in activation energies for H- versus D-abstraction was found to be significantly greater than the theoretical difference of 1.3kcal/mol. These results clearly reflected the smaller tunneling probability of the heavier deuterium atom. 

Qualitatively the results are explained in the following way. Although the transferring deuterium atom does not introduce a primary isotope effect due to zero-point energy differences into ko/ko, there is less tunnelling when deuterium is transferred than when hydrogen is transferred. Therefore, the tunnel correction to the secondary /c°//cd is small relative to that for k /k. Thus, the experimental results are in agreement with the results of the model calculations. 

C Kinetic Deuterium Isotope Effects and H Atom Tunnelling in the... 

B. C. Garrett and D. G. Truhlar, Critical tests of variational transition state theory and semiclassical tunneling methods for hydrogen and deuterium atom transfer reactions and use of the semiclassical calculations to interpret the overbarrier and tunneling dynamics, 7. Phys. Chem. 95 10374 (1991). 

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. 

Complications that arise with this simple reaction are twofold. First, because of the low mass of the hydrogen atom its movement frequently exhibits non-classical behavior, in particular quantum-mechanical tunneling, which contributes significantly to the observed kinetic isotope effect, and in fact dominates at low temperature (Section 6.3). Secondly, in reaction 10.2 protium rather than deuterium transfer may occur ... 

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