Ultraviolet photolysis of CH2I2 in methanol: O-H insertion and HI elimination reactions to form a dimethoxymethane product

Ultrafast structural dynamics of in-cage isomerization of diiodomethane in solution

Despite extensive studies on the isomer species formed by photodissociation of haloalkanes in solution, the molecular structure of the precursor of the isomer, which is often assumed to be a vibrationally hot isomer formed from the radical pair, and its in-cage isomerization mechanism remain elusive. Here, the structural dynamics of CH₂I₂ upon 267 nm photoexcitation in methanol were probed with femtosecond X-ray solution scattering at an X-ray free-electron laser. The determined molecular structure of the transiently formed species that converts to the CH₂I–I isomer has the I–I distance of 4.17 Å, which is longer than that of the isomer (3.15 Å) by more than 1.0 Å and the mean-squared displacement of 0.45 Ų, which is about 100 times larger than those of typical regular chemical bonds. These unusual structural characteristics are consistent with either a vibrationally hot form of the CH₂I–I isomer or the loosely-bound radical pair (CH₂I˙⋯I˙).

The structural dynamics of in-cage isomerization of CH₂I₂ and the unusual structure of the loosely-bound isomer precursor were unveiled with femtosecond X-ray liquidography (solution scattering).

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.

Transient isomers in the photodissociation of bromoiodomethane

The photochemistry of halomethanes is fascinating for the complex cascade reactions toward either the parent or newly synthesized molecules. Here, we address the structural rearrangement of photodissociated CH₂IBr in methanol and cyclohexane, probed by time-resolved X-ray scattering in liquid solution. Upon selective laser cleavage of the C-I bond, we follow the reaction cascade of the two geminate geometrical isomers, CH₂I-Br and CH₂Br-I. Both meta-stable isomers decay on different time scales, mediated by solvent interaction, toward the original parent molecule. We observe the internal rearrangement of CH₂Br-I to CH₂I-Br in cyclohexane by extending the time window up to 3 μs. We track the photoproduct kinetics of CH₂Br-I in methanol solution where only one isomer is observed. The effect of the polarity of solvent on the geminate recombination pathways is discussed.

Spin-Orbit Ab Initio Investigation of the Ultraviolet Photolysis of Diiodomethane

The UV photodissociation (<5 eV) of diiodomethane (CH(2)I(2)) is investigated by spin-orbit ab initio calculations. The experimentally observed photodissociation channels in the gas and condensed phases are clearly assigned by multi-state second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space-state interaction potential energy curves. The calculated results indicate that the fast dissociations of the first two singlet states of CH(2)I(2) and CH(2)I--I lead to geminate-radical products, CH(2)I (.)+I((2)P(3/2)) or CH(2)I (.)+ I*((2)P(1/2)). The recombination process from CH(2)I--I to CH(2)I(2) is explained by an isomerization process and a secondary photodissociation reaction of CH(2)I--I. Finally, the study reveals that spin-orbits effects are significant in the quantitative analysis of the electronic spectrum of the CH(2)I--I species.

Modeling SN2 Reactions in Methanol Solution by ab Initio Calculation of Nucleophile Solvent−Substrate Clusters

[Structure: see text]. Ab initio calculations were used to study the S(N)2 reactions of the CH3OCH2I molecule with a methoxide ion (CH3O-) and a methanol molecule by systematically building up the reaction system with explicit incorporation of the methanol solvent molecules. For the reaction of CH3OCH2I with a methoxide ion, the explicit incorporation of the methanol molecules to better solvate the methoxide ion led to an increase in the barrier to reaction. For the reaction of CH3OCH2I with a methanol molecule, the explicit incorporation of the methanol molecules led to a decrease in the barrier to reaction because of an inclination of this reaction to proceed with the nucleophilic displacements accompanied by proton transfer through the H-bonding chain. The H-bonding chain served as both acid and base catalysts for the displacement reaction. A ca. 10(15)-fold acceleration of the methanol tetramer incorporated S(N)2 reaction was predicted relative to the corresponding methanol monomer reaction. The properties of the reactions examined are discussed briefly.