Unimolecular dissociation reverse

Peroxy radicals ROO are key species in the mechanisms of oxidative and combustion systems. At the same time they have been among the most difficult radicals to study experimentally. There are only limited or no thermochemical information available for unsaturated alkylperoxy and hydroperoxide species. An explanation for this paucity of data could be the fact that these species are unstable and short-lived, and therefore difficult to study and characterise by experimental methods. The difficulty arises in part from the lability of these radicals towards reversible unimolecular dissociation into R + O2 and then to reversible isomerization into hydroperoxy alkyl radicals ROOH, both of these reactions occurring at comparable rates at temperatures below 450 K [2]. 

Klippenstein S J 1992 Variational optimizations in the Rice-Ramsperger-Kassel-Marcus theory calculations for unimolecular dissociations with no reverse barrier J. Chem. Rhys. 96 367-71... 

The maximum rate for the reverse reaction (unimolecular dissociation, s ) is via separation by diffusion of the two molecules... 

The above discussion shows that several possible pathways for the interconversion of sulfur rinp exist. However, none of these alone can explain all the experimental observations. It therefore seems likely that several of them are effective simultaneously. Unimolecular dissociation reactions as discussed under (a) and (d) will dominate at high temperatures due to the increase in entropy. At lower temperatures, however, bimolecular reactions like the dimerization (c) may be most important, at least in case of the small rings (Sg, S, Sg) whose unimolecular dissociation is strongly endothermic. Larger rings will probably decompose according to mechanism (d), which in a way is the reversal of the dimerization (c). 

A and B is converted to the internal (vibrational) energy of the association product. By analogy, the highly excited species formed is denoted C. Because it is highly unstable, C may unimolecularly dissociate by the forward direction of reaction 9.101, with rate constant kd, or if C collides with another species M, it could be stabilized via the reverse of reaction 9.100, forming the product C. 

A single substance may rearrange or decompose. These are considered to be the two unimolecular processes, although the mechanism for each may involve another species (a catalyst). Reversible decomposition (dissociation) is often assisted by the presence of other molecules (a solvent, perhaps). Finally, the molecules of a single substance may oligomerize. 

Multifrequency Quantum Rice-Ramsperger-Kassel (QRRK) is a method used to predict temperature and pressure-dependent rate coefficients for complex bimolecular chemical activation and unimolecular dissociation reactions. Both the forward and reverse paths are included for adducts, but product formation is not reversible in the analysis. A three-frequency version of QRRK theory is developed coupled with a Master Equation model to account for collisional deactivation (fall-off). The QRRK/Master Equation analysis is described thoroughly by Chang et al. [62, 63]. 

Only at very high temperatures (T dSOOK) and [S02]/[Ar] 0-5.10 could the unimolecular dissociation SO2 -> SO + O be observed in shock waves. At lower temperatures and higher [S02]/[Ar] secondary bimolecular reactions were obviously important a complex mechanism is evident, but cannot yet be uniquely resolved. An exceptionally high rate is reported for the reverse reaction O -1- SO SO2 = [Ar] 3-2 x lO cm mol" s- = [Ar]. 8-7 x... 

The recombination A + B AB is the reverse reaction of the unimolecular dissociation of AB. The principle of detailed balancing ties both reactions together by the thermodynamic equilibrium constant ku... 

Optimizations in the Rice-Ramsperger-Kassel-Marcus Theory Calculations For Unimolecular Dissociations With No Reverse Barrier. 

At the present time a number of gaseous unimolecular reactions are known. The view that none exist, although it appeared plausible for a time, has now been definitely abandoned. Nevertheless, unimolecular reactions are rather exceptional and appear to be confined to molecules of rather complex structure. It is possible that the decomposition of diatomic molecules into atoms at high temperatures is unimolecular but more probable that it is bimolecular, the reverse reaction of recombination being termolecular. Thus the rate of dissociation of chlorine would be k1 [Cl2]2 while the rate of recombination of the atoms would be 2 [Cl]2 [Clg], according to the Herzfeld theory (p. 111). 

Figure 9 shows the temperature dependence of the recovered kinetic rate coefficients for the formation (k bimolecular) and dissociation (k unimolecular) of pyrene excimers in supercritical CO2 at a reduced density of 1.17. Also, shown is the bimolecular rate coefficient expected based on a simple diffusion-controlled argument (11). The value for the theoretical rate constant was obtained through use of the Smoluchowski equation (26). As previously mentioned, the viscosities utilized in the equation were calculated using the Lucas and Reichenberg formulations (16). From these experiments we obtain two key results. First, the reverse rate, k, is very temperature sensitive and increases with temperature. Second, the forward rate, kDM, 1S diffusion controlled. Further discussion will be deferred until further experiments are performed nearer the critical point where we will investigate the rate parameters as a function of density. 

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