Kinetics, chemical unimolecular reactions

Both unimolecular and bimolecular reactions are common throughout chemistry and biochemistry. Binding of a hormone to a reactor is a bimolecular process as is a substrate binding to an enzyme. Radioactive decay is often used as an example of a unimolecular reaction. However, this is a nuclear reaction rather than a chemical reaction. Examples of chemical unimolecular reactions would include isomerizations, decompositions, and dis-associations. See also Chemical Kinetics Elementary Reaction Unimolecular Bimolecular Transition-State Theory Elementary Reaction... 

Type I Photoinitiators Unimolecular Photoinitiators. These substances undergo an homolytic bond cleavage upon absorption of light. The fragmentation that leads to the formation of radicals is, from the point of view of chemical kinetics, a unimolecular reaction. The number of initiating radicals formed upon absorption of one photon is termed the quantum yield of radical formation... 

As reactants transfonn to products in a chemical reaction, reactant bonds are broken and refomied for the products. Different theoretical models are used to describe this process ranging from time-dependent classical or quantum dynamics [1,2], in which the motions of individual atoms are propagated, to models based on the postidates of statistical mechanics [3], The validity of the latter models depends on whether statistical mechanical treatments represent the actual nature of the atomic motions during the chemical reaction. Such a statistical mechanical description has been widely used in imimolecular kinetics [4] and appears to be an accurate model for many reactions. It is particularly instructive to discuss statistical models for unimolecular reactions, since the model may be fomuilated at the elementary microcanonical level and then averaged to obtain the canonical model. 

Quack M and Tree J 1976 Unimolecular reactions and energy transfer of highly excited molecules Gas Kinetics and Energy Transfer mo 2, oh 5, ed P G Ashmore and R J Donovan (London The Chemical Society) pp 175-238 (a review of the literature published up to early 1976)... 

The study of the rates of chemical reactions is called kinetics. Chemists study reaction rates for many reasons. To give just one example, Rowland and Molina used kinetic studies to show the destructive potential of CFCs. Kinetic studies are essential to the explorations of reaction mechanisms, because a mechanism can never be determined by calculations alone. Kinetic studies are important in many areas of science, including biochemistry, synthetic chemistry, biology, environmental science, engineering, and geology. The usefulness of chemical kinetics in elucidating mechanisms can be understood by examining the differences in rate behavior of unimolecular and bimolecular elementary reactions. 

STOICHIOMETRIC NUMBER Stoichiometry of elementary reactions, CHEMICAL KINETICS MOLECULARITY UNIMOLECULAR BIMOLECULAR TRANSITION-STATE THEORY ELEMENTARY REACTION STOKE S SHIFT... 

Recombination Reactions, Blackwell Scientific, Oxford, 1990. (c) J. I. Steinfeld, J. S. Francisco, and W. L. Hase, Chemical Kinetics and Dynamics, Prentice Hall, Englewood Cliffs, 1989. (d) P. J. Robinson and K. A. Holbrook, Unimolecular Reactions, Wiley-Interscience, Bristol, 1972. 

Note that the mean value of the stochastic representation is the deterministic result, showing that the two representations are consistent in the mean. We shall see later that this is true only for unimolecular reactions. The stochastic model, however, also gives higher moments and so fluctuations can now be included in chemical kinetics. One sees that the stochastic approach is to chemical kinetics as statistical thermody-... 

Within the last 15 years, the field of chemical kinetics, particularly gas phase kinetics, has undergone considerable change. One might almost be tempted to term this revolutionary. The nature of this change is that a large body of quantitative data has been accumulated about elementary unimolecular reactions and bimolecular reactions involving radicals,... 

In the traditional surface science approach the surface chemistry and physics are examined in a UHV chamber at reactant pressures (and sometimes surface temperatures) that are normally far from the actual conditions of the process being investigated (catalysis, CVD, etching, etc.). This so-called pressure gap has been the subject of much discussion and debate for surface science studies of heterogeneous catalysis, and most of the critical issues are also relevant to the study of microelectronic systems. By going to lower pressures and temperatures, it is sometimes possible to isolate reaction intermediates and perform a stepwise study of a surface chemical mechanism. Reaction kinetics (particularly unimolecular kinetics) measured at low pressures often extrapolate very well to real-world conditions. 

Since A, B, and C are regions in the phase space of single closed system, the transitions between A and represent a unimolecular reaction or isomerization, rather than a general reaction in the sense of chemical kinetics. Unlike some unimolecular reactions, (e.g the decomposition of diatomic molecules) the molecular dynamics system of eq. 1 will be assumed to have sufficiently many well-coupled degrees of freedom that transitions between reactant and product regions occur spontaneously, without outside interference. 

Experimental rate constants, kinetic isotope effects and chemical branching ratios for the CF2CFCICH3-do, -d, -d2, and -d2 molecules have been experimentally measured and interpreted using statistical unimolecular reaction rate theory.52 The structural properties of the transition states needed for the theory have been calculated by DFT at the B3PW91 /6-31 G(d,p/) level. 

In chemical kinetics, we learn that an elementary unimolecular reaction... 

The initial decomposition chemistry involves unimolecular reactions. This was the conclusion of the first gas-phase kinetics study [84] and has been repeatedly confirmed by subsequent bulb and shock-tube experiments [85, 86]. That first study used shock heating to induce thermal decomposition [84], The data were interpreted in terms of simple C-N bond fission to give CH2 and N02. A more extensive and definitive shock-tube study was reported by Zhang and Bauer in 1997 [85]. Zhang and Bauer presented a detailed kinetics model based on 99 chemical reactions that reproduced their own data and that of other shock-tube experiments [84, 86]. An interesting conclusion is that about 40% of the nitromethane is lost in secondary reactions. 

With this replacement of the strong collider assumption now commonplace, the term RRKM theory has become largely synonymous with quantum TST for unimolecular reactions, and we use this terminology here. The foundations of RRKM theory have been tested in depth with a wide variety of inventive theoretical and experimental studies [9]. While these tests have occasionally indicated certain limitations in its applicability, for example to timescales of a picosecond or longer, the primary conclusion remains that RRKM theory is quantitatively valid for the vast majority of conditions of importance to chemical kinetics. The H + O2 HO2 OH + O reaction is an example of an important reaction where deviations from RRKM predictions are significant [10, 11]. The foundations of RRKM theory and TST have been aptly reviewed in various places [7, 9, 12-15]. Thus, the present chapter begins with only a brief... 

The most highly developed theories in chemical kinetics are those describing unimolecular reactions. Reference has already been made to them in some detail in Chapter 2 where their use in analysing data on radical... 

In these circumstances, where routine kinetic measurements are uninformative and direct measurements of the product-forming steps difficult, comparative methods, involving competition between a calibrated and a non-calibrated reaction, come into their own. Experimentally, ratios of products from reaction cascades involving a key competition between a first-order and a second-order processes are measured as a function of trapping agent concentration. Relative rates are converted to absolute rates from the rate of the known reaction. The principle is much the same as the Jencks clock for carbenium ion lifetimes (see Section 3.2.1). However, in radical chemistry Newcomb prefers to restrict the term clock to a calibrated unimolecular reaction of a radical, but such restriction obscures the parallel with the Jencks clock, where the calibrated reaction is a bimolecular diffusional combination with and the unknown reaction a pseudounimolecular reaction of carbenium ion with solvent. Whatever the terminology, the practical usefulness of the method stems from the possibility of applying the same absolute rate data to all reactions of the same chemical type, as discussed in Section 7.1. 

An important concept in chemical kinetics is molecularity of a reaction or the number of particles (molecules, atoms, ions, radicals) participating in it. Most common are bimolecular reactions, unimolecular reactions being also encountered. In very rare cases termolecular reactions may be observed as well. Reactions of higher molecularity are unknown, which is due to a very low probability of a simultaneous interaction of a larger number of molecules. Consequently, our further considerations will be confined to the examination of uni- and bimolecular reactions. On the other hand, the reactions of a termolecular character, whose kinetic equations have a number of interesting properties, are sometimes considered. As will appear, a termolecular reaction may be approximately modelled by means of a few bimolecular reactions. For an elementary reaction its molecularity is by definition equal to the order whereas for a complex reaction the molecularity generally has no relation whatsoever to the reaction order or the stoichiometry. 

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... 

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