Fate of transient isomer of CH2I2: Mechanism and origin of ionic photoproducts formation

초록

Diiodomethane, CH2I2, in a polar solvent undergoes a unique photoinduced reaction whereby I2? and I3? are produced from the photodissociation of CH2I2, unlike for other iodine-containing haloalkanes that commonly produce only iodine atoms upon photodissociation. Despite many experimental and theoretical studies, the origin and mechanism of this unique photo-chemical behavior have not been understood yet.1?3 Here, using time-resolved X-ray solution scattering (TRXSS), we revisit the photodissociation mechanism of CH2I2 in methanol and determine the structures of all transient species and photoproducts involved in the photodissociation of CH2I2. According to the analysis of our TRXSS data, at the earliest time delay (150 ps) after photoexcitation at 267 nm, iso-CH2I-I, CH2I? radical, and I? radical are formed. While previous optical spectroscopic and TRXSS studies proposed that homolysis, heterolysis, or solvolysis of iso-CH2I-I 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. In the present work, based on the molecular structures of transient intermediates and reaction kinetics revealed by our TRXSS experiment, we reveal that I2? and I3? are formed via heterolysis of iso-CH2I?I in the photodissociation of CH2I2 in methanol. In addition, with the aid of quantum chemical calculations on a series of iodine compounds that photogenerate isomers containing an I?I bond, we demonstrate that the high polarity of iso-CH2I?I and its subtle interaction with the polar solvent is responsible for the unique photochemistry of CH2I2 in the polar solvent, that is, the formation of the negative ionic species.