Multiple‐Well, multiple‐path unimolecular reaction systems. II. 2‐methylhexyl free radicals

Abstract

Vibrationally excited 2-methylhexyl radicals formed by shock wave activation or by chemical activation can isomerize by multiple pathways to form any of six stable isomers, can fragment by multiple C-H and CC bond fission pathways, and can be collisionally stabilized. Master equation simulations of chemical activation and of shock wave activation are used to explore the generic behavior of this complicated coupled system. Selecting the argon pressure in chemical activation systems that produce the 2-methyl-1-hexyl radical isomer (1) can control the yield of specific isomers. Shock heating of 1 also shows a highly regular sequence of isomer formation. This regular behavior is because the first isomerization steps are faster than subsequent steps. Other radical isomers, such as 2-methyl-3-hexyl (3), do not show such regular behavior, because the first isomerization step is slower than subsequent steps. Incubation and unimolecular rate-constant fall-off are observed in the shock wave simulations. The unimolecular rate-constant fall-off for the coupled system produces low-pressure limiting rate constants proportional to [M] n , where n can be greater than unity. The fact that n can be greater than unity is a natural feature of multichannel coupled unimolecular reaction systems, but detection of the effect in experiments may be very demanding.

Theoretical study of the thermochemistry and the kinetics of the SF Cl (x= 0–5) series

“…This is a useful and reasonably accurate approximation, suitable in the absence of detailed knowledge of the transition state [39]. The above k(E) expression is converted to k(E) = A 1 [(E À E 1 + a(E À E 1 )E Z )/(E + a(E)E Z )] sÀ1 by using the Whitten-Rabinovitch approximation for the density of states [40].…”

Relative Rates of Hydrogen Shift Isomerizations Depend Strongly on Multiple-Structure Anharmonicity

“…Nevertheless, it is interesting to consider the independent-reaction falloff effects for the isomerizations because they are independent of mechanism and can be considered as well-defined lower bounds on the competing-reaction pressure-dependent falloff effects (i.e., as giving approximate upper bounds on the pressure-dependent rate constants). Because the independent-reaction assumption becomes more severe at low pressure, we will only discuss the pressure dependence for pressures of 1 atm and above.…”

“…The independent-reaction assumption is entirely correct for all the results discussed so far, which correspond to the high-pressure limit, but it is an extra assumption for calculating falloff effects because the pressure dependence of a given reaction can be strongly influenced by the reactions that compete with it. Therefore the rate constants must eventually be studied in the context of an overall reaction mechanism modeled by a master equation. − For the case of competing isomerization reactions, each competing reaction depletes the same reservoir of reactive states of a given reactant, and this lowers the reaction rates as compared to those calculated by the independent-reaction assumption . Since the effect depends on the relative threshold energies of the competing reactions, it can also affect the pressure-dependent branching ratios .…”

“…Therefore the rate constants must eventually be studied in the context of an overall reaction mechanism modeled by a master equation. − For the case of competing isomerization reactions, each competing reaction depletes the same reservoir of reactive states of a given reactant, and this lowers the reaction rates as compared to those calculated by the independent-reaction assumption . Since the effect depends on the relative threshold energies of the competing reactions, it can also affect the pressure-dependent branching ratios . Nevertheless, it is interesting to consider the independent-reaction falloff effects for the isomerizations because they are independent of mechanism and can be considered as well-defined lower bounds on the competing-reaction pressure-dependent falloff effects (i.e., as giving approximate upper bounds on the pressure-dependent rate constants).…”

Ab initio study on alkyl radical decomposition and alkyl radical addition to olefins

“…We found kinetic parameters for these reactions which are generally useful for the modeling of complex systems [11]. Recently, a consistent and accurate set of heats of formation for alkyl radicals was calculated by ab initio methods [12,13,14,15].…”