Activation nuclear tunneling through

Equation (4) presents a commonly used ET model that is a special case of Eq. (1) [5,35,36,38,39,53-58]. Here only a single vibrational (nuclear) mode is active, and it is in the high-temperature (or classical) limit. In this model all the reactants that become products pass over the top of the reaction barrier (an activated process), and none of the reaction occurs by nuclear tunneling through the barrier. However, not all of the reactants that reach the top of the barrier actually become products many simply relax without reacting (nonadiabatic or weak-interaction limit). 

In Sect. 7, we raised the question of what were the chemical stimuli to which the reactivity indices defined in Sect. 6, the softness kernels, were presumed to be the responses, our seventh issue. Now there are various broad categories of reactions to be considered, unimolecular, bimolecular, and multimolecular. The former occur via thermal activation over a barrier, tunneling through the barrier, or some combination of both. There is no stimulus, and the softness kernels defined as responses of the electron density to changes in external or nuclear potential are irrelevant. For the study of unimolecular reactions, one needs only information about the total energy in the relevant configuration space of the molecule. 

Electron tunneling would be very fast and depend negligibly on temperature if only electronic motion is involved. An equally important contributor to the rate is the nuclear factor, which depends on how much the molecular structure is changed during ET. The nuclear factor depends on vibrational structure and hence on temperature. In fact, the nuclei find an activation barrier in their motion due to ET. Also, in this case we may talk about tunneling through a barrier, or nuclear tunneling. 

In any case, nuclear tunneling cannot be avoided. The nuclear tunneling effect depends on the fact that at a low temperature the nuclei do not have enough energy to pass the activation barrier. Instead, tunneling through the barrier by the nuclei becomes relatively more important. 

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