Hydrogen atom tunnelling

Quantum-chemical calculations of PES for carbonic acid dimers [Meier et al. 1982] have shown that at fixed heavy-atom coordinates the barrier is higher than 30kcal/mol, and distance between O atoms is 2.61-2.71 A. Stretching skeleton vibrations reduce this distance in the transition state to 2.45-2.35 A, when the barrier height becomes less than 3 kcal/mol. Meier et al. [1982] have stressed that the transfer is possible only due to the skeleton deformation, which shortens the distances for the hydrogen atom tunneling from 0.6-0.7 A to 0.3 A. The effective tunneling mass exceeds 2mn-... 

These data led to the model already described several times above. The enzyme executes a search for a tunneling sub-state, apparently 13 kcaFmol in energy above the principal state from this state the hydrogen atom tunnels with no further vibrational excitation. Probably motion of the secondary center is coupled into the tunneling coordinate. The result is large, temperature-independent primary and secondary isotope effects in the context of an isotope-independent activation energy. 

Kuznetsov, A.M. and Ulstrup, J. (1999). Proton and hydrogen atom tunnelling in hydrol3dic and redox enzyme catalysis. Can. J. Chem. 77, 1085-1096... 

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). 

VARIATIONAL TRANSITION STATE THEORY CALCULATIONS OF CONCERTED HYDROGEN ATOM TUNNELING IN WATER CLUSTERS AND FORMALDEHYDEAVATER CLUSTERS... 

Fig. 34. PES cuts for the reaction (6.19) C H3 -H C HjOH - C" -H C HjOH for the C-C distance corresponding to the hydrogen atom tunneling distance 1.0 A. 1,1 are the diabatie Morse terms, 2 the corresponding adiabatic potential, 3 the onedimensional parabolie barrier fitted to the experimental data, 4 potential along MEP for the exchange gas-phase reaction CH3 -E CH4 - CH4 -H CH3 [Hipes and Kupperman 1986].

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