Unimolecular reactions model-specificity

The interest of studies on relaxation mechanisms of van der Waals complexes is general they may be considered as models for a larger class of unimolecular reactions. The specificity of van der Waals systems relies on the exceptionally weak coupling between intra- and inter-molecular modes, much weaker than typical intra-state coupling in molecules. The time scales for IVR and VP processes are therefore significantly longer and more convenient for real-time experiments. 

Waite B A and Miller W H 1980 Model studies of mode specificity in unimolecular reaction dynamics J. Chem. Phys. 73 3713-21... 

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... 

In chapter 7 the statistical adiabatic channel model (SACM) (Quack and Troe, 1974, 1975) was described for calculating unimolecular reaction rates. This theory assumes the reaction system remains on the same diabatic potential energy curve while moving from reactant to products. Two parameters, a and (3 are used to construct model diabatic potential curves. The unimolecular rate constant, at fixed E and 7, for forming products with specific energy , (e.g., a specific vibrational energy in one of the fragments) is... 

At first glance it seems paradoxical to treat unimolecular reactions, in which a single molecule is apparently involved in reaction, in terms of a collision theory based on pairwise interactions. Indeed, we have developed a rather specific picture of a chemical reaction from the hard-sphere collision model, in which bonds are formed rather than broken and in which the energetics of reaction are represented in terms of relative kinetic energy. 

W. H. Miller I would like to ask Prof. Schinke the following question. Regarding the state-specific unimolecular decay rates for HO2 — H + O2, you observe that the average rate (as a function of energy) is well-described by standard statistical theory (as one expects). My question has to do with the distribution of the individual rates about die average since there is no tunneling involved in this reaction, the TST/Random Matrix Model used by Polik, Moore and me predicts this distribution to be x-square, with the number of decay channels being the cumulative reaction probability [the numerator of the TST expression for k(E)] how well does this model fit the results of your calculations ... 

Consideration of bound-state dynamics affords one advantage not shared by systems undergoing reaction or decay. Specifically, since formal ergodic conditions require a compact phase space, ideal chaotic systems exist for bound systems but not for bimolecular collisions or unimolecular decay. Studies of these ideal bound systems therefore provide a route for analyzing statistical behavior in circumstances where the system is fully characterized. Furthermore, these ideal system results can be compared with the behavior of model molecular systems to assess the degree to which realistic systems display chaotic relaxation. 

Our approach to the study of the departure from equilibrium in chemical reactions and of the "microscopic theory of chemical kinetics is a discrete quantum-mechanical analog of the Kramers-Brownian-motion model. It is most specifically applicable to a study of the energy-level distribution function and of the rate of activation in unimolecular (dissociation Reactions. Our model is an extension of one which we used in a discussion of the relaxation of vibrational nonequilibrium distributions.14 18 20... 

The term unimolecular does not mean that no other molecules are Involved in the rate-limiting step, since the reaction is not observed in the absence of solvent. Therefore, the designation polymolecular was used by Steigman, J. Hammett, L. P. /. Am. Chem. Soc. 1937,59,2536, and the term termolecular (based on a specific model of solvent interaction) was suggested by Swain, G. C. /. Am. Chem. Soc. 1948, 70,1119. 

In this chapter, we give an account of our recent MD and theoretical analysis of electron transfer" (ET) and SnI ionization" reactions in RTILs. Specifically, we consider the unimolecular ET of a model diatomic reaction complex in l-butyl-3-methyldicyanamide (BMI" DCA ) and ionization of 2-chloro-2-methylpropane in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PEg ). The influence of the RTIL environment on free energetics and dynamics of these reactions is described with attention paid to its similarities to and differences from the conventional polar solvents. The MD results for barrier crossing dynamics on reaction kinetics are analyzed via the Grote-Hynes (GH) theory and compared with the transition state theory (TST) and Kramers theory predictions. 

The rate constants for unimolecular and solvolytic reactions generally show a monotonic decrease (i.e., micellar inhibition)" - or a monotonic increase (i.e., micellar catalysis) - or insensitivity (i.e., micellar-independent rate)"- " to an increase in micellar concentration. There seems to be no exception to this generalization and, if there is one, it is owing to some specific chemical or physical reasons. For example, the nnimolecular decarboxylation of 6-nitrobenzisox-azole-3-carboxylate ion (1) in CTABr micelles is enhanced by the salts of hydrophilic anions and slowed by the salts of hydrophobic anions, whereas salts such as sodium tosylate increased reaction rate when in low concentration, and retarded it when in high concentration. The first theoretical model, known as the... 

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