Chemical reaction barrierless

B. Kohler 1 would like to ask two questions to Prof. Zewail. First, in your investigation of the electron transfer reaction in a benzene- complex, the sample trajectory calculations you showed appear to suggest that the charge transfer step may induce vibrationally coherent motion in h-. Have you tested this possibility experimentally My second question concerns your intriguing results on a tautomerization reaction in a model base-pair system. In many of the barrierless chemical reactions you have studied, you have been able to show that an initial coherence created in the reactant molecules is often observable in the products. In the case of the 7-azaindole dimer system your measurements indicate that reaction proceeds quite slowly on the time scale of vibrational motions (such as the N—H stretch) that are coupled to the reaction coordinate. What role do you think coherent motion might play in reactions such as this one that have a barrier ... 

Seki, K., Bagchi, B., TacMya, M. Dynamics of barrierless and activated chemical reactions in a dispersive medium within the fractional diffusion equation approach. J. Phys. Chem. B 112(19), 6107-6113 (2008). http //dx.doi.org/10.1021/Jp076753q... 

Addition is even more facile in radical -b radical reactions due to the usually barrierless formation (on the singlet-coupled surface) of a chemical bond... 

In summary, we may say that the NBO/NRT description of partial proton transfer in the equilibrium H-bonded complex(es) is fully consistent with the observed behavior along the entire proton-transfer coordinate, including the transition state. At the transition state the importance of partial co valency and bond shifts can hardly be doubted. Yet the isomeric H-bonded complexes may approach the TS limit quite closely (within 0.2 kcal mol-1 in the present example) or even merge to form a single barrierless reaction profile (as in FHF- or H502+). Hence, the adiabatic continuity that connects isomeric H-bond complexes to the proton-transfer transition state suggests once more the essential futility of attempting to describe such deeply chemical events in terms of classical electrostatics. 

Such radiation sources all have sufficient energy to break the chemical bonds of molecules in the ISM and thence produce both reactive radicals and ions capable of inducing further chemistry. In the gas phase much ofthe chemistry in the ISM is driven by ion-molecule reactions (Fig. 3). Such reactions are barrierless that is they require no energy to start the reaction rather once the reactants are brought together the reaction appears to occur spontaneously. Such barrierless reactions are also prevalent if one of the reacting species is a free radical (e.g. the hydroxyl radical OH). Such reactions can therefore occur at low temperatures, indeed it has been noted that the reaction rate may actually increase at low temperatures. 

Protection of products from active species in the active plasma zone seriously restricts the yield of non-thermal plasma-chemical NO synthesis. The most important fast barrierless reverse reaction, leading to destraction of NO inside the active discharge zone, is... 

It is not always, of course, that a barrierless (j3 = 1) or activationless (/3 = 0) process produces slopes corresponding, respectively, to 60 mV and 00. As can be seen, for example, from Eq. (52), under conditions where a preceding equilibrium step is involved, these values of jS may result in slopes corresponding to 0.030 and 0.060 V, respectively. Conversely, it is equally possible that in certain cases, such slopes which are sometimes interpreted as indicative of a slow chemical step, correspond, in fact, to the limiting cases of the electrochemical reaction per 5e, considered above. 

Figure 15.13 Effect of temperature on the rate constant of reaction H + OH - H2O modelled by the Noyes relationship (eqn (15.1)) (solid line). A satisfactory agreement with experimental values ( ) " provides the test for the barrierless chemical step.

As explained above, the formation of new molecules on the surface is a combination of several independent processes, especially when the stable, detectable products are the result of multiple reactions. The barriers for the individual processes are very hard to disentangle and this is only possible with additional information from, for instance, TPD experiments. For this reason, usually only a qualitative measure for the efficiency of the reaction is given. Common assessments are (1) effectively barrierless, (2) with a barrier that allows the reaction to proceed at 10 K, (3) with a larger barrier so that reaction does not proceed at this temperature, (4) thermally activated, or proceeds through quantum chemical tuimelling. 

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