Two-channel thermal unimolecular decomposition of alkyl iodides

The competition between the C-I bond fission and the four-center HI elimina tion 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 absol ute rate constants were measured at temperatures of 950 - 1100 K. The branc hing fractions for C-I bond fission channels were determined for all isomer s of C-3 and C-4 alkyl iodides at temperatures of 950-1400 K. A drastic cha nge 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 t he threshold energy for the HI elimination channel from primary to secondar y (by similar to 14 kJ mol(-1)) and from secondary to tertiary (by similar to 20 kJ mol(-1)) iodides. The alpha-CH3 substituent effect to the activati on 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 the ir temperature dependence. A simple interpretation for the alpha-CH3 substi tuent effect is presented in terms of the avoided intersection between ioni c dissociation (RI --> R+ + I-) surface and the repulsive surface of HI app roach to the double bond.