Unimolecular Reaction—General Features

If the activated complex were constrained, as is the case when the isotopic reactants are atoms, the opposite would be true an inverse isotope effect is expected. 2  

In Chapter 5 we examined the phenomenology of unimolecular reactions and their pressure-dependent order of reaction. The simple mechanistic model (5.18) predicted the rate law 

Laidler, Chemical Kinetics 2nd ed. (New York McGraw-Hill, 1965), p. 97. 

These ideas on energy transfer can be formalized by considering the rate of formation and the rate of destruction of excited molecules with energies between and c + de. Excited molecules are formed via collisions. If n is the concentration of unexcited A molecules, the total collision rate as computed in (2.21) is yn l2, where y, assuming the molecules to be hard spheres of diameter rf, 

A factor of 2 is introduced because in the collision process either of the A molecules might be excited. 

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Just as the E2 mechanism shares features of the Sn2 mechanism, the El mechanism shares features of the Sn 1 reaction. The initial step is formation of a carbocation intermediate through loss of the leaving group. This slow step becomes the rate-determining step for the whole reaction, i.e. the El mechanism is unimolecular. In general terms, the reaction can be represented as follows. 

Initiated by the chemical dynamics simulations of Bunker [37,38] for the unimolecular decomposition of model triatomic molecules, computational chemistry has had an enormous impact on the development of unimolecular rate theory. Some of the calculations have been exploratory, in that potential energy functions have been used which do not represent a specific molecule or molecules, but instead describe general properties of a broad class of molecules. Such calculations have provided fundamental information concerning the unimolecular dissociation dynamics of molecules. The goal of other chemical dynamics simulations has been to accurately describe the unimolecular decomposition of specific molecules and make direct comparisons with experiment. The microscopic chemical dynamics obtained from these simulations is the detailed information required to formulate an accurate theory of unimolecular reaction rates. The role of computational chemistry in unimolecular kinetics was aptly described by Bunker [37] when he wrote The usual approach to chemical kinetic theory has been to base one s decisions on the relevance of various features of molecular motion upon the outcome of laboratory experiments. There is, however, no reason (other than the arduous calculations involved) why the bridge between experimental and theoretical reality might not equally well start on the opposite side of the gap. In this paper... results are reported of the simulation of the motion of large numbers of triatomic molecules by... 

Although this discussion of gas-phase reactions can hardly be considered comprehensive, many important and general features of gas-phase mechanisms have been considered. Unimolecular decompositions are discussed in some detail in the next chapter, and some approaches to elementary reactions in the gas phase are considered in Chapters 5 and 6. 

In conclusion, it appears that the general features of unimolecular reactions are reasonably well described by microscopic treatments involving various molecular parameters. Since there is still room for improvement in the agreement between theory and experiment, it is not unlikely that we shall see further refinements of existing theories or new theoretical approaches to the problem along with additional experimental work. A more extensive discussion of unimolecular decompositions is given by Robinson and Holbrook [10]. 

I begin with a brief survey of the general properties of thermal unimolecular reactions, noting the salient features which any minimal theory should at least illuminate, if not explain. 

A number of authors have pointed out that spectra acquired using various desorption techniques have many features in common (11-14). Generally the ions detected are even electro ionj molecular ions formed by protonation or addition of NH-, Na, etc., and fragment ions formed by elimination of neutral molecules. It is not clear to what extent pyrolysis and solvolysis reactions augment the contributions of unimolecular gas phase decompositions to spectra obtained using the various desorption techniques. Glucuronic Acid Conjugate Desorption... 

The book is divided into seven distinct parts and these are subdivided into individual chapters. In Parts I -3 we present the general principles that underpin the operation of lasers, the key properties of laser radiation, the main features of the various laser sources, and an overview of the most commonly used laser spectroscopic techniques, together with the instrumentation and methods for data acquisition. In Parts 4-6 we address the principles of unimolecular, bimolecular, cluster and surface reactions, which have been probed, stimulated or induced by laser radiation. In the final part, Part 7, we summarize a range of practical laser applications in industry, environmental studies, biology and medicine, many of which are already well established and in routine use. 

These results are in general agreement with the values or limits determined from conventional kinetic studies by Halpem and Wong and Tolman et al. It was found that the dimer and the dihydrogen adduct react with CO rather slowly (Jl 2 s ) and by a unimolecular process. A remarkable feature of these results is the large rate constant for displacement of ethylene by CO, as shown in the following reaction ... 

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