Fate of transient isomer of CH₂I₂: Mechanism and origin of ionic photoproducts formation unveiled by time-resolved x-ray liquidography

Diiodomethane, CH₂I₂, in a polar solvent undergoes a unique photoinduced reaction whereby I₂^- and I₃^- are produced from its photodissociation, unlike for other iodine-containing haloalkanes. While previous studies proposed that homolysis, heterolysis, or solvolysis of iso-CH₂I-I, which is a major intermediate of the photodissociation, can account for the formation of I₂^- and I₃^-, there has been no consensus on its mechanism and no clue for the reason why those negative ionic species are not observed in the photodissociation of other iodine-containing chemicals in the same polar solvent, for example, CHI₃, C₂H₄I₂, C₂F₄I₂, I₃^-, and I₂. Here, using time-resolved X-ray liquidography, we revisit the photodissociation mechanism of CH₂I₂ in methanol and determine the structures of all transient species and photoproducts involved in its photodissociation and reveal that I₂^- and I₃^- are formed via heterolysis of iso-CH₂I-I in the photodissociation of CH₂I₂ in methanol. In addition, we demonstrate that the high polarity of iso-CH₂I-I is responsible for the unique photochemistry of CH₂I₂.