Formation of radical cations of polyiodomethanes: a pulse radiolysis study

Abstract

The transient optical absorption band (λ_max= 380 nm, t_½= 23 µs and ε= 2010 dm³ mol^–1 cm^–1) produced on pulse radiolysis of N₂O-saturated aqueous solution of CH₂I₂(pH 6.0) has been assigned to the OH adduct radical, CH₂I₂·OH. In acidic aqueous solutions, CH₂I₂⁺(λ_max= 380 and 570 nm, t_½= 3 µs and ε₅₇₀= 2 150 dm³ mol^–1 cm^–1) has been identified as the intermediate. The rate constants for the reaction of OH radicals with CH₂I₂ to form CH₂I₂·OH and CH₂I₂⁺ have been determined to be 2.1 × 10⁹ and 6.0 × 10⁹ dm³ mol^–1 s^–1 respectively. Cl₂^– has been found to oxidize CH₂I₂ to CH₂I₂⁺ with a bimolecular rate constant of 1.7 × 10⁸ dm³ mol^–1 s^–1. CH₂l₂⁺ is a good oxidant and oxidizes I^–, Br^–, and methyl disulphide with high rate constants close to the diffusion limit. I₂ is the main stable end product formed on γ-radiolysis and its yield is equal to ½ G(-CH₂I₂). The transient optical absorption bands observed on pulse radiolysis of CH₂I₂ and CHI₃ in 1,2-dichloroethane have been assigned to CH₂I₂⁺(λ_max= 380 and 590 nm) and CHI₃⁺(λ_max= 400 and 610 nm).