Tunneling in chemical reactions

Now that It has been made evident that tunneling is predicted by quantum mechanics, and that there are a number of physical manifestations of It, what about the particular area of chemical reactions  

ACS Symposium Series American Chemical Society Washington, DC, 1975. 

The potential energy of interaction for the reactants In a hypothetical reaction such as 

I am deliberately evading here some subtle questions, such as  

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Quantum tunnelling in chemical reactions can be visualised in terms of a reaction coordinate diagram (Figure 2.4). As we have seen, classical transitions are achieved by thermal activation - nuclear (i.e. atomic position) displacement along the R curve distorts the geometry so that the... 

Quantum tunneling in chemical reactions 28 Quantum tunneling in solution reactions 29... 

I am not familiar with the results you mentioned. Looking for the biochemical studies that have directly confirmed our observation of molecular tunneling in chemical reactions, one should mention the investigations of the dynamics of ligand rebinding to heme proteins performed by H. Frauenfelder, I. Gunsalus, and their colleagues at the University of Illinois (Urbana, 111.) [see, e.g., Biochemistry, 14, 5355 (1975) and Science, 192, 1002 (1976)]. 

One interesting analogy should be noted here. It is well known that the exponential factor that determines the rate of tunneling contains the product d. JmE, where d is the barrier width, E is its height, and m is the mass of tunneling particle. In chemical cases in ours and American works, d 10 8-5cm, m 30, E 0, 1 eV. In the spontaneous fission of nuclei, d 10 12 cm, m 100, E 106 eV. Thus the spontaneous fission of nuclei and molecular tunneling in chemical reactions can be treated to some extent as quite similar phenomena. 

GENERAL FEATURES OF NUCLEAR TUNNELING IN CHEMICAL REACTIONS... 

Bourgin [51] and Roginsky and Rosenkewitsch [52] were the first to pay attention to the possibility of the tunnel mechanism of chemical reactions. They did so quite soon after the creation of quantum mechanics. At that time, the main features of this phenomenon were also understood on the qualitative level. Later on, a large number of theoretical and experimental works were dedicated to more detailed studies on tunnel effects in chemical reactions. This field has attracted the interest of scientists up to the present time. A comprehensive review of the history and the present state of investigations of nuclear tunneling in chemical reactions can be found in the recently published monographs by Bell [53] and Goldanskii et al. [54],... 

Manifestations of nuclei tunneling in chemical reactions in gaseous, liquid, and solid phases are consecutively considered in Sects. 4.2-4.5. Also discussed in this chapter are (1) manifestations of nuclear tunneling in the vibrational spectra of ammonia-type molecules (Sect. 4.6), (2) electron tunneling in gas-phase chemical reactions of atom transfer (the so-called "harpoon reactions, Sect. 4.2), and (3) tunneling of hydrated electrons in the reactions of their recombination with some inorganic anions in aqueous solutions (Sect. 4.4). 

Convincing experimental evidence of nuclei tunneling in the course of chemical reactions has been obtained over the last two decades in studies of chemical reactions at low temperatures (below 100 K, as a rule) in condensed media. Nuclear tunneling in chemical reactions, however, usually occurs only over short (less than 1 A) distances. Elementary estimations show that in chemical reactions practically no tunneling of nuclei can be observed at distances exceeding the sum of the van der Waals radii of reacting molecules. This is due to the large mass of the atomic nuclei. 

Chem. Phys. 1995, 370(3), Special Issue Tunneling in Chemical Reactions,... 

The two chief experimental criteria for tunneling in chemical reactions are an abnormal isotope effect (the tunnel effect is much more pronounced for hydrogen than for deuterium), which does not concern us here, and a curved Arrhenius plot. The reason for this is that the effect becomes most marked at low temperatures, when the fraction of systems which are able to cross the barrier becomes considerably higher than that calculated from classical considerations. As a result, the rate decreases with decreasing temperature less than expected, and the Arrhenius plot becomes concave upward. We cannot go into the quantum-mechanical details, and refer the reader to the literature on the subject. (See, e.g., Refs. 2b, 23, 77, 99, 105.)... 

It appears that the relations (261.Ill) and (265.III) are appropriate for an experimental test of the role of nuclear tunneling in chemical reactions. It should be taken into consideration, however, that these relations imply either an approximate non-adiabatic or a complete dynamic separability of the reaction coordinate. 

Week, P.F. and Balakrishnan, N., Heavy atom tunneling in chemical reactions Study of H + LiF collisions, J. Chem. Phys., 122, 234310, 2005. 

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