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, ⟨ΔE⟩_down = 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 ⟨ΔE⟩_down = 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.