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.

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The competition between the C-I bond fission and the four-center HI elimination in the thermal unimolecular decomposition of C 3 -C 4 alkyl iodides has been investigated at temperatures of 950-1400 K and pressures around 1 atm by a shock tube technique. The concentration of iodine atoms was followed by atomic resonance absorption spectrometry. For primary iodides, the absolute rate constants were measured at temperatures of 950-1100 K. The branching fractions for C-I bond fission channels were determined for all isomers of C 3 and C 4 alkyl iodides at temperatures of 950-1400 K. A drastic change in the branching fraction for the C-I bond fission channel was observed from primary iodides (0.6-0.9) to secondary iodides (0.2-0.4), and further to tertiary iodide (<0.05), which was mainly ascribed to the lowering of the threshold energy for the HI elimination channel from primary to secondary (by ∼14 kJ mol -1 ) and from secondary to tertiary (by ∼20 kJ mol -1 ) iodides. The R-CH 3 substituent effect to the activation energy was in good accordance with previous investigations. The observed temperature dependence of the branching fraction could not be explained by the simple high-pressure limit treatment, and an RRKM analysis showed that the proper treatment of the mutual effect of two dissociation channels is essentially important to reproduce the observed branching fractions and their temperature dependence. A simple interpretation for the R-CH 3 substituent effect is presented in terms of the avoided intersection between ionic dissociation (RI f R + + I -) surface and the repulsive surface of HI approach to the double bond.

The site-specific branching fractions for the reaction of O(3P) atom with C3H8 [→ OH + n-C3H7 (1a); → OH + i-C3H7 (1b)] and the subsequent OH-radical reaction with C3H8 [→ H2O + n-C3H7 (2a); → H2O + i-C3H7 (2b)] have been studied experimentally at temperatures 593, 944, and 1130 K. Since it was difficult to separate the O-atom reaction from the rapid subsequent OH-radical reaction, the sum of the branching fractions for (1b) and (2b) was determined by measuring the yield of i-C3H7 radicals. Two methods are presented and have been tested for the discrimination of alkyl isomers. At low temperature (593 K), i-C3H7 radical was directly and selectively detected with photoionization mass spectrometry by utilizing the difference of ionization potentials of n-C3H7 and i-C3H7 radicals. At higher temperatures (944 and 1130 K), the yield of i-C3H7 radical was determined by using a laser photolysis−shock tube apparatus from the yield of H atoms, which are produced from the thermal decomposition of i-C3H7 radicals. It was confirmed experimentally that the i-C3H7 radical exclusively decomposes to H + C3H6 while the n-C3H7 radical mainly decomposes to CH3 + C2H4 and the minor decomposition pathway (→ H + C3H6) contributes little (<5%) in the present experimental conditions. By subtracting the reported branching fraction for (2b) [Droege, A. T.; Tully, F. P. J. Phys. Chem. 1986, 90, 1949], the branching ratio for the O(3P) atom reaction was evaluated to be k 1a/k 1b = 2.5 exp(−8.9 kJ mol-1/RT) [593−1130 K], which well agrees with the transition-state theory calculation by Cohen and Westberg [Int. J. Chem. Kinet. 1986, 18, 99].

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