Decomposition and Vibrational Relaxation in CH3I and Self-Reaction of CH3 Radicals

Vibrational relaxation and dissociation of CH3I, 2−20% in krypton, have been investigated behind incident shock waves in a diaphragmless shock tube at 20, 66, 148, and 280 Torr and 630−2200 K by laser schlieren densitometry. The effective collision energy obtained from the vibrational relaxation experiments has a small, positive temperature dependence, ⟨ΔEdown = 63 × (T/298)0.56 cm−1. First-order rate coefficients for dissociation of CH3I show a strong pressure dependence and are close to the low-pressure limit. Restricted-rotor Gorin model RRKM calculations fit the experimental results very well with ⟨ΔEdown = 378 × (T/298)0.457 cm−1. The secondary chemistry of this reaction system is dominated by reactions of methyl radicals and the reaction of the H atom with CH3I. The results of the decomposition experiments are very well simulated with a model that incorporates methyl recombination and reactions of methylene. Second-order rate coefficients for ethane dissociation to two methyl radicals were derived from the experiments and yield k = (4.50 ± 0.50) × 1017 exp(−32709/T) cm3 mol−1 s−1, in good agreement with previous measurements. Rate coefficients for H + CH3I were also obtained and give k = (7.50 ± 1.0) × 1013 exp(−601/T) cm3 mol−1 s−1, in reasonable agreement with a previous experimental value.