Vacuum ultra-violet photodissociation of alkyl iodides. Kinetics of the reaction H + ICH3→ HI + CH3

Abstract

The vacuum u.v. photodissociation of alkyl iodides has been investigated using flash photolysis techniques. At wavelengths between 140 and 170 nm, the primary process is selective and results in C—H rather than C—I scission: it is also sensitive to deuterium isotope substitution. At wavelengths > 170 nm, the quantum yield of C—H scission is < 10^–2. The selective C—H scission has allowed an investigation of energy disposal in the reactions H + ICH₃→ HI + CH₃(3a) D + ICD₃→ DI + CD₃(3b) as well as their kinetics. The rate constant k_3a has been measured as (5.8₆± 0.2₉)× 10¹² cm³ mol^–1 s^–1 at (293 ± 2)K, by monitoring the rate of development of the Rydberg absorption bands of HI. No evidence of transient vibrational (or electronic) excitation of the methyl radicals produced in (3) or through the photodissociation of methyl iodide at > 170 nm, has been observed. These, and related observations, can be understood if, in photodissociation or bimolecular iodine atom transfer, the methyl radical attains a near planar conformation before recoiling from the neighbouring iodine atom.