J. Phys. Chem. A 1999, 103, 2723-2733
2723
Thermal Decomposition and Isomerization Processes of Alkyl Radicals Noboru Yamauchi, Akira Miyoshi,* Keishi Kosaka, Mitsuo Koshi, and Hiroyuki Matsui Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: NoVember 17, 1998; In Final Form: February 2, 1999
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 primaryto-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 f 2-C5H11) ) 4.88 × 108T 0.846 exp(-19.53 [kcal mol-1]/RT) s-1 and k∞(1-C6H13 f 2-C6H13) ) 6.65 × 107T 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.
Introduction Thermal decomposition of alkyl radicals plays an important role in the combustion of hydrocarbons. It is one of the important elementary reactions in the oxidation processes of alkanes, and it competes with the reaction with O2, especially at high temperatures and under fuel-rich conditions. Recently, Gutman, Slagle, and co-workers have investigated the thermal decomposition of C2 to C4 alkyl radicals at low temperatures (99.9999%; H2, >99.9999%), Wako (I2, 99.9%; CH3I, >99%; n-C3H7I, >97%; i-C3H7I, >99%), and Tokyo Kasei (C2H5I, >99%; n-C4H9I, >98%; s-C4H9I, >97%; i-C4H9I, >97%; t-C4H9I, >95%; 1-C5H11I, >98%; 1-C6H13I, >98%). All reagents (alkyl iodides) were purified by trap-totrap distillation. Ar was purified by passing through a cold trap (-140 °C). Indicated error limits for the experimental results are at two standard deviations level throughout the paper. Results A. Thermal Decomposition of C3-C4 Alkyl Radicals. Figure 1a shows the observed time profiles of I- and H-atom concentrations in the thermal decomposition of n-C3H7I. The observed concentration of H atoms was much smaller than that of I atoms, which is a measure of the initial concentration of n-C3H7 radicals, indicating that the thermal decomposition of n-C3H7 produces little H atoms. On the other hand, as shown
(4)
In order to minimize the effect of this reaction, the initial concentration was kept low especially for i-C3H7I experiments, although this effect is still slightly visible in Figure 1b which was observed under the initial concentration as low as 1 ppm. For n-C3H7I, the branching fraction for HI elimination (1b) is smaller (0.2-0.3) and thus higher initial concentration was applied in order to confirm the very small yield of H atoms. The H-atom yields from the thermal decomposition of two isomeric propyl radicals at different temperatures are summarized in Figure 2. The H-atom yield from the thermal decomposition of i-C3H7 radical was unity within the experimental error limit, 1.01 ( 0.27, and was found to be independent of temperature. This indicates the dominance of the C-H bond fission in the thermal decomposition of i-C3H7 radical,
i-C3H7 + M f C3H6 + H + M ∆H298 ) 35.7 kcal mol-1 (5) This is the only possible reaction channel except for the isomerization to n-C3H7. The H-atom yield from n-C3H7 radical was found to be much smaller,
