Unimolecular reactions theory

Unimolecular reaction theory with specific reference to ions has been reviewed fairly recently [548, 883 see also 759, 837]. 

The absorption of a quantum of light by a molecule in the gas phase may initiate a unimolecular decomposition or rearrangement process. The potentially intimate relationship between photochemistry and nonequilibrium unimolecular reaction theory has yet to be realized, since most of these photoprocesses take place on electronically excited poten-... 

The unimolecular reaction theory was successfully applied to the rate of dissociation of ions created in mass spectrometry. In this situation, the rate constant k for the dissociation is dependent on the internal energy of the ion. A simplified form (an approximation) for the expression of the rate constant is, in this case,... 

Figure 2.11 Typical discrepancy between unimolecular reaction theory, equation (2-39), and experimental data.

The previous analysis is of interest not only for general mode transition problems, but also for providing a useful model for unimolecular reaction theory. 

Consider again the potential given by equation (10). If its behaviour is examined at large p values, it is seen that it has three symmetric saddles at height (6X ) at p=X, and eventually goes to minus infinity. Therefore all the states which it supports are actually metastable, and they will eventually decay by quantum mechanical tunnelling in other words, they are typical quantum mechanical resonances, to which we may associate a width r and a lifetime =fi/r. This model has already been considered [35,36] for unimolecular reaction theory, where the resonance li fetime is most naturally related to the inverse of the unimolecular rate constants k=x... 

Because of an editorial policy discouraging the presentation of experimental results in both graphical and tabular form, the primary rate constant data for the thermal isomerisation of cyclopropane in the fall-off region [53.P2] are only available in thesis form. One might have expected these results to have been superseded by now, but that has not happened, and Sowden s rather inaccessible thesis [54.S] remains the only source of these key data. In view of their continuing importance in the testing of unimolecular reaction theories, I am reproducing those results here (and also those of the cyclobutane reaction) for the convenience of future users. 

Historically, the theory of the termolecular reaction mentioned in the previous paragraph has been developed through the unimolecular reaction theory. This paragraph describes unimolecular decomposition reacticms in some detail. The chemical formula for the unimolecular decomposition reactions corresponding to the Lindemann mechanism can be shown as... 

The RRKM theory reproduces experimental values weU,and is now recognized as an established unimolecular reaction theory. The calculation by the RRKM theory has been applied to atmospheric reactions, for example, OH + CO (Sect. 5.2.3), OH + NO2 + M (Sect 5.2.4) for obtaining the theoretical rate constants based... 

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