Unimolecular kinetics

Bunker D L and Hase W L 1973 On non-RRKM unimolecular kinetics molecules in general and CH3NC in particulars Chem. Rhys. 59 4621-32... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Vinyl cations have been postulated as intermediates in the aqueous acetone solvolysis of 257 (214). Unimolecular kinetics were observed, and the sole products of solvolysis were the corresponding amides, 258. A rho value of about p = —1.2 was observed for the effect of substituents X and p = —.7 for substituents Y in 257. The relatively small value of rho for the aryl substituents... 

Upon addition of a solution of sulfuric acid in D20 the reaction of A-acetoxy-A-alkoxyamides obeys pseudo-unimolecular kinetics consistent with a rapid reversible protonation of the substrate followed by a slow decomposition to acetic acid and products according to Scheme 5. Here k is the unimolecular or pseudo unimolecular rate constant and K the pre-equilibrium constant for protonation of 25c. Since under these conditions water (D20) was in a relatively small excess compared with dilute aqueous solutions, the rate expression could be represented by the following equation ... 

Stable isotope analysis of Earth, Moon, and meteorite samples provides important information concerning the origin of the solar system. 8lsO values of terrestrial and lunar materials support the old idea that earth and moon are closely related. On the other hand three isotope plots for oxygen fractionation in certain meteoric inclusions are anomalous. They show unexpected isotope fractionations which are approximately mass independent. This observation, difficult to understand and initially thought to have important cosmological implications, has been resolved in a series of careful experimental and theoretical studies of isotope fractionation in unimolecular kinetic processes. This important geochemical problem is treated in some detail in Chapter 14. 

In order to measure the magnitude of the chemical interactions between various ions and buffer gases, approaches that are based on the measurements of either equilibrium or rate constants for ionic processes can be envisioned. An example of a kinetic method is described in the following. The unimolecular kinetic process known as thermal electron detachment (TED) for negative ions (NT -> M + e), should be particularly sensitive to a chemical effect of the buffer gas. This is because the rate of TED will be given by = constant x where the electron... 

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. 

The formation of a coordinated reduced nitrobenzoato ligand radical (or, in spectroscopic terms, a MLCT excited state) has been found in the reaction of e-q with pentaaminocobalt(III) complexes containing a mono- or di-nitrobenzoato ligand272. Such a coordinated radical disappears via unimolecular kinetics, which represents ligand-to-metal intramolecular electron transfer with formation of Co2+... 

Further isomerization of m-xylene as well as transalkylation of trimethylbenzene and toluene to form m-xylene can occur. Evidence for the bimolecular transalkylation mechanism was provided by observation of a peak at m/e 109 in the mass spectra for CD3 substitution of toluene. These data rule out unimolecular 1,2-methyl shifts as the sole means of formation of xylenes. The higher the A1 content of the ultrastable faujasite the greater the extent of bimolecular transalkylation. These observations have significant implications for unimolecular kinetic models that have been proposed as well as reported activation energies and turnover frequencies. 

Modem Aspects of Diffusion-Controlled Reactions Low-temperature Combustion and Autoignition Photokinetics Theoretical Fundamentals and Applications Applications of Kinetic Modelling Kinetics of Homogeneous Multistep Reactions Unimolecular Kinetics, Part 1. The Reaction Step Kinetics of Multistep Reactions, 2nd Edition... 

In most of the studies to date, the rate data reflects a significant amount of induced decomposition. The data in large part do not represent a unimolecular process, but instead, a composite of the reactions associated with induced decomposition which are described above. Although our objective is to present unimolecular kinetic data, we have deviated from this objective here since the bulk of the kinetic data for hydroperoxides includes induced decomposition. 

In the conventional treatment of unimolecular kinetics [34,55] one evaluates this average in the limit of weak perturbation strength by expanding the... 

The foregoing general conclusions of Kramers were first verified in a specific case by Eyring and Zwolinskj, 19 who took into account the quantized nature of the molecular levels in an elementary way. They compared the results of the exact integration of typical unimolecular kinetic equations to the results based on the assumption that equilibrium was maintained throughout the reaction. By making assumptions which are supposed to cover the extremes likely to be encountered, they found that the nonequilibrium rate may deviate by no more than 20 per cent from its equilibrium rate. 

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... 

Direct dynamics simulations, in which the methodology of classical trajectory simulations is coupled to electronic structure, have had and will continue to have an enormous impact on the use of computational chemistry to develop [111,112] the theory of unimolecular kinetics. In these simulations the derivatives of the potential, required for numerically integrating the classical trajectory, are obtained directly from electronic stmcture theory without the need for an analytic PES. Direct dynamics is particularly important for studying the unimolecular dynamics of molecules with many degrees of freedom, for which it is difficult to construct an accurate analytic PES. 

As reactants transform to products in a chemical reaction, reactant bonds are broken and reformed 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 postulates 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 unimolecular 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 formulated at the elementary microcanonical level and then averaged to obtain the canonical model. 

In the statistical description of unimolecular kinetics, known as Rice-Ramsperger-Kassel-Marcus (RRKM) theory [4,7,8], it is assumed that complete IVR occurs on a timescale much shorter than that for the unimolecular reaction [9]. Furthermore, to identify states of the system as those for the reactant, a dividing surface [10], called a transition state, is placed at the potential energy barrier region of the potential energy surface. The assumption implicit in RRKM theory is described in the next section. 

As discussed by Slater (1959) and Bunker (1964), the lifetime distribution P(t, E) provides a link between microscopic and macroscopic unimolecular kinetics. The lifetime distribution is related to N(r, E), Eq. (8.22), according to... 

Volume 39 Unimolecular Kinetics, Part 1. The Reaction Step... 

The principal experimental methods for measuring bond dissociation energies in polyatomic molecules have been described in detail by Cottrell (35). Two methods in particular have proved fruitful in recent years, namely the toluene carrier gas technique introduced by Szwarc (167) to investigate the unimolecular kinetics of pyrolytic decomposition reactions, and the electron impact method, as applied by Stevenson (165) and widely used since by several investigators. These methods do not usually give very sharp D values, and the number of well-authenticated ( 2... 

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