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 I-2(-) and I-3(-) 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 I-2(-) and I-3(-), 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, I-3(-), and I-2. 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 I-2(-) and I-3(-) 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.