Tunneling effects energy transfer

Many computational studies in heterocyclic chemistry deal with proton transfer reactions between different tautomeric structures. Activation energies of these reactions obtained from quantum chemical calculations need further corrections, since tunneling effects may lower the effective barriers considerably. These effects can either be estimated by simple models or computed more precisely via the determination of the transmission coefficients within the framework of variational transition state calculations [92CPC235, 93JA2408]. 

DR. EPHRAIM BUHKS (University of Delaware) I would like to ask your opinion about a possible interpretation of proton transfer in terms of nuclear tunneling effects. Might it be possible that as the energy of the vibrational modes becomes very large, the classical rate theory might not work ... 

Because the dependence on the distance of separation of donor and acceptor is similar for both energy transfer by the exchange effect and for electron transfer by tunnelling, this section can be abbreviated, the more so since the subject has been reviewed recently by Rice and Pilling [39]). 

The first applications (230-234) concerned a study of the dependence of the tunneling effect on the curvature of the reaction path, of an energy transfer among different vibrational modes in the course of a reaction, and of the evaluation of microcanonical rate constants for study-case reactions, HCN - CNH, H2CO -> H2 + CO, H2CC -> HC=CH, and proton transfer in malonaldehyde. 

The characteristic Arrhenius plots predicted by this approach have been reported on a number of occasions for reactions involving the transfer of a proton to or from a carbon atom in the rate-determining step (Hulett, 1964 Caldin and Kasparian, 1965 and references there cited). It is also encouraging to find that the difference between the rates observed at the lower temperatures and those calculated at the same temperatures from the linear portion of the Arrhenius plot are always consistent with reasonable values for the barrier width, 2a. However, concurrent reaction via two different mechanisms having different activation energies (see p. 123) has been considered to account for some of the observations previously interpreted in terms of the tunnel effect (Jones, 1965). 

---