Kinetics of Iodine Atom Recombination between 300 and 1164°K

Abstract

The recombination of iodine atoms in the presence of argon and I2 has been studied between 300 and 1164°K. The rate constants, k r Ar and k r I 2 , are (in liters2 mole−2·second−1) Temperature (° K ) 298 473 573 864 1064 1164 k r Ar × 10 −8 24 8.7 6.6 4.8 4.1 4.4 k r I 2 × 10 −10 140 11 6 0.9 At 1064°K, the rate constants k r I 2 and k r I were estimated to be equal to or less than 10k r Ar . These results were interpreted in terms of the radical–molecule complex (RMC) mechanism [J. K. K. Ip and G. Burns, J. Chem. Phys. 51, 3414 (1969) and J. A. Blake, R. J. Browne, and G. Burns, J. Chem. Phys. 53, 3320 (1970)]. This mechanism predicts that the interaction potential between an iodine atom and an argon atom is four to nine times deeper than the potential between argon and xenon, which is iodine's neighbor in the periodic table. The potential depth for the I–Ar complex was estimated to be between 1.2 and 2.8 kcal/mole, depending upon which assumptions were used in the RMC mechanism. The interaction potential between I and I2 was estimated to be 5.5 kcal/mole deep. The present calculations suggest that the three‐body reaction probabilities for 2I+Ar→I2+Ar reaction are in the range between 0.2 and 0.02, dependent upon assumptions used in the RMC mechanism. Although the RMC mechanism yields a satisfactory agreement with the experiment, at highest temperatures, the “energy‐transfer” mechanism, I + I ⇋ I 2 *, I 2 *+ Ar → I 2 + Ar , appears to contribute to the over‐all recombination. At 1064°K, approximately one‐quarter of recombination seems to proceed by this route. Present recombination rate data overlap the shock wavedissociation rate data by some 300°C. Above 1000°K, the recombination rate constants calculated from shock wave data are generally higher, and display a steeper negative temperature dependence, than the flash photolysis recombination rate constants, obtained at the same temperature. These discrepancies display a pattern similar to those already reported for bromine [J. K. K. Ip and G. Burns, J. Chem. Phys. 51, 3425 (1969)] and chlorine [G. Burns and R. J. Browne, J. Chem. Phys. 53, 3318 (1970)]. An analysis of thermal effects, which are unimportant at room temperature, but tend to dominate recombination above 1000°K, is also given.