Multiple-well, multiple-path unimolecular reaction systems. II. 2-methylhexyl free radicals

Vibrationally excited 2-methylhexyl radicals formed by shock wave activatio n or by chemical activation can isomerize by multiple pathways to form any of six stable isomers, can fragment by multiple C-H and C-C bond fission pa thways, and can be collisionally stabilized. Master equation simulations of chemical activation and of shock wave activation are used to explore the g eneric behavior of this complicated coupled system. Selecting the argon pre ssure in chemical activation systems that produce the 2-methyl-1-hexyl radi cal isomer (1) can control the yield of specific isomers. Shock heating of 1 also shows a highly regular sequence of isomer formation. This regular be havior is because the first isomerization steps are faster than subsequent steps. Other radical isomers, such as 2-methyl-3-hexyl (3), do not show suc h regular behavior, because the first isomerization step is slower than sub sequent steps. Incubation and unimolecular rate-constant fall-off are obser ved in the shock wave simulations. The unimolecular rate-constant fall-off for the coupled system produces low-pressure limiting rate constants propor tional 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 unimolecul ar reaction systems, but detection of the effect in experiments may be very demanding. (C) 2001 John Wiley & Sons, Inc.