Barrierless association reactions

Actually, the TST expression only holds if all molecules that pass from the reactant over the TS go on to the product, the ideal situation to the ion-molecule barrierless association reaction. The TST assumption is that no recrossing occurs for a given temperature, i.e., all molecules passing through the dividing surface will go on to form the product. 

In recent years, there has been tremendous progress in the treatment of barrierless association reactions with strictly loose transition states. 

Cyanide ions react with the soft alkyl halides in SN2 reactions and with the hard carbocations in SnI reactions to give, almost always, the nitrile 4.27, which is thermodynamically preferred. Isonitrile products are formed along with the nitrile products when the cation is so reactive that the rate has reached the diffusion-controlled limit, and the reversible reaction that would equilibrate the products is too slow. One consequence when reactions are as fast as this is that there is a barrierless combination of ions, and selectivity is not then controlled by the kinetic factors associated with the principle of hard and soft acids and bases. 

Just as with the thiocyanate ion 73, the cyanide ion 76 reacts with soft alkyl halides in SN2 fashion and with hard carbocations in SN1 fashion to give, almost always, the nitrile 77, which is thermodynamically more stable than the corresponding isonitrile 78. The isonitrile product is formed along with the nitrile product when (a) the cation is so very reactive that the rate of reaction reaches d iIf u s ion -con tro 11 ed limit and (b) the reversible reaction that equilibrates the isonitrile and nitrile products is very slow. Since the reaction of a cyanide ion with a carbocation falls in the domain of ion-to-ion reaction, it is indeed very fast. For such a barrierless combination of ions, the kinetic factors associated with the HSAB principle are not applicable. 

This variational version of TST can be used to calculate temperature-dependent rate constants for the barrierless reactions [3-6]. In VTST, the TS state is located at the free energy maximum along the reaction path, and the association rate constant is given by... 

The PST calcrrlations of the association rates for the ion-molecule reactions can be carried out using programs such as VariFlex [86]. Though the main aim of this program is the calculation of rates for barrierless reactions, it also allows convenient estimates of the rates using PST. The program employs Monte Carlo phase space integration to evaluate the statistical functions. 

We can thus expect in principle that every electrode (and not only an electrode) process can be realized in three forms barrierless, ordinary, and activationless. If we ignore the complications associated with the influence of other stages of the reaction, the... 

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