Thermal Decomposition and Isomerization Processes of Alkyl Radicals

Abstract

The thermal decomposition and isomerization processes of C3−C4 alkyl radicals, 1-C5H11, and 1-C6H13 have been investigated by using a shock-tube apparatus coupled with atomic resonance absorption spectrometry (ARAS). Isomeric alkyl radicals were generated by the thermal decomposition of respective alkyl iodides. Branching fractions for the competitive pathways (C−C bond cleavage, C−H bond cleavage, and isomerization) have been determined by following the hydrogen-atom concentration by ARAS. In the investigated temperature range (900−1400 K), for all alkyl radicals, the energetically favored C−C bond cleavage was found to dominate over the C−H bond cleavage. The 1,2 or 1,3 isomerization reaction was found to be minor in C3 and C4 alkyl radicals. On the other hand, the results for 1-C5H11 and 1-C6H13 radicals clearly show the occurrence of 1,4 and 1,5 isomerization reactions. From an RRKM analysis of the present result and the previous lower temperature data, with consideration of the tunneling effect, the threshold energies for 1,4 and 1,5 primary-to-secondary isomerization reactions were evaluated to be 21.5 ± 1.2 and 14.6 ± 1.2 kcal mol-1, respectively. The high-pressure limit rate constants for the isomerization processes were evaluated as k ∞(1-C5H11 → 2-C5H11) = 4.88 × 108 T 0.846 exp(−19.53 [kcal mol-1]/RT) s-1 and k ∞(1-C6H13 → 2-C6H13) = 6.65 × 107 T 0.823 exp(−12.45 [kcal mol-1]/RT) s-1 for the temperature range 350−1300 K. Even under relatively high-pressure conditions (∼1 atm), the falloff effect was shown to be important for multichannel dissociation systems. The nonequilibrium effect in the thermal decomposition of energized alkyl radicals formed in the high-temperature reaction system, which has been first suggested by Tsang et al. [J. Phys. Chem. 1996, 100, 4011] was discussed. The possible effect of the tunneling in the isomerization reactions was discussed in comparison with previous lower temperature data.

Computational studies on the reactions of 3‐butenyl and 3‐butenylperoxy radicals

“…4. For most of the alkyl radicals, C C bond fission reactions are preferred to C H bond fission where they are possible [28]. This is also true for the butyl radical shown in Fig.…”

“…4 indicate the unimolecular isomerization reactions and further reactions of the isomerization products. As has been well known for butyl radicals [28], the isomerization reactions are unimportant due to the high barriers, 161 kJ mol −1 for both 3,1-and 2,1-hydrogen shifts ( * h3 and * h2). However, for 3-butenyl, two hydrogen shifts ( * h2 and * h4) and internal addition channels ( * a3 and * a4) were found with barriers less than or close to those of the C H and C C bond fission channels ( * ch and * cc).…”

“…For channels * h4 and * a4, the rate coefficients are not so large, but the small equilibrium constants indicate the validity of the assumption of neglecting these two channels. For this type of system, multiple-well and multiple-channel treatment [28,31] may be necessary, but, for this case, the assumption applied here can be easily validated by the equilibrium constants and high barrier for other channels of the isomerization products. Finally, falloff rate parameters for the unimolecular reactions of butyl and 3-butenyl are summarized in Table V.…”

“…Investigations based on the variation of the RRKM/master equation analyses [28,31] for the multichannel multiple-well system are underway in our research group and will be published in the near future. Since little has been known for the internal addition reaction of alkenylperoxy radicals in alkene oxidation, some simplified analyses based on the TST calculations are presented here for qualitative discussion.…”

Multiple‐Well, multiple‐path unimolecular reaction systems. I. MultiWell computer program suite

“…Each isomer can decompose via C -C and C -H bond fission to produce a total of 14 distinct sets of fragmentation products [4]. In toto, there are 49 unimolecular reaction channels, all of which are treated specifically by using MultiWell.…”