Isomerization from bimolecular transfer

Characterization of ion structures by bimolecular reactions, in which an ion is allowed to react with a neutral gas of known structure and the ionic products are analysed by mass spectrometry, depends on isomeric species having distinctive reactivities which reflect the functional group(s) that are present. This method is conceptually analogous to the use of structure-specific test reagents in classical solution chemistry. Sometimes a group may be transferred to a particular ion from the gas (methylene transfer is commonly encountered) on other occasions, hydrogen transfer is monitored. The latter is conveniently combined with isotopic labelling. 

For another contrast between SAC and GAC we need only refer you back to the two Z/E isomer-izations earlier in the chapter. Isomerization of the diene is GAC—protonation at carbon is the slow step—and isomerization of the allylic alcohol is SAC. What we didn t tell you earlier was that the GAC reaction has a normal kinetic isotope effect of k(H)lk(D) = 2.5 and a negative entropy of activation AS = -36 J mol-1 K-1—just what we should expect for a bimolecular reaction involving rate-determining proton transfer from oxygen to carbon. Notice that the intermediate cation is tile same whichever the route only the ways of getting there, including the rate-determining steps, are different. 

The above examples illustrate the important types of solution reactions, especially from the point of view of analytical chemistry. But it is not an exhaustive list. For example, another type of solution reaction which is of interest to theorists is the isomerization reaction [G3]. In this chapter, the theory of the electron transfer process is considered in detail. Some of the results from this treatment can also be applied to other bimolecular processes. More details follow in the later sections of this chapter. 

The Ir(bpy)2(bpy-C3)2 + complex has been used to photosensitize the conversion of norbornadiene to quadricyclene in a bimolecular process [96,97], The limiting quantum yield of the photosensitized isomerization is 0.72 and the Stern-Volmer rate constant is 1.4 x 10"8 M 1s 1. The more likely mechanism for this reaction is thought to be partial electron transfer from the excited state of the complex to norbornadiene [97],... 

Activated reactions take several different forms. In thermally activated systems, most of the reactions that have been studied via molecular dynamics are isomerizations, dissociations/recombinations, simple bimolecular reactions (such as atom replacement), or electron transfer (which we shall not review here). The simulation of activation in these systems requires techniques different from those for nonthermally activated systems, of which the most common class studied (in a variety of condensed phases) is photodissociation. 

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