Picosecond Time-Resolved Resonance Raman Study of CH2I−I Produced after Ultraviolet Photolysis of CH2I2 in CH3OH, CH3CN/H2O and CH3OH/H2O Solutions

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.

Taking the plunge: chemical reaction dynamics in liquids

The dynamics of chemical reactions in liquid solutions are now amenable to direct study using ultrafast laser spectroscopy techniques and advances in computer simulation methods. The surrounding solvent affects the chemical reaction dynamics in numerous ways, which include: (i) formation of complexes between reactants and solvent molecules; (ii) modifications to transition state energies and structures relative to the reactants and products; (iii) coupling between the motions of the reacting molecules and the solvent modes, and exchange of energy; (iv) solvent caging of reactants and products; and (v) structural changes to the solvation shells in response to the changing chemical identity of the solutes, on timescales which may be slower than the reactive events. This article reviews progress in the study of bimolecular chemical reaction dynamics in solution, concentrating on reactions which occur on ground electronic states. It illustrates this progress with reference to recent experimental and computational studies, and considers how the various ways in which a solvent affects the chemical reaction dynamics can be unravelled. Implications are considered for research in fields such as mechanistic synthetic chemistry.

Perspective: Bimolecular chemical reaction dynamics in liquids

Bimolecular reactions in the gas phase exhibit rich and varied dynamical behaviour, but whether a profound knowledge of the mechanisms of isolated reactive collisions can usefully inform our understanding of reactions in liquid solutions remains an open question. The fluctuating environment in a liquid may significantly alter the motions of the reacting particles and the flow of energy into the reaction products after a transition state has been crossed. Recent experimental and computational studies of exothermic reactions of CN radicals with organic molecules indicate that many features of the gas-phase dynamics are retained in solution. However, observed differences may also provide information on the ways in which a solvent modifies fundamental chemical mechanisms. This perspective examines progress in the use of time-resolved infra-red spectroscopy to study reaction dynamics in liquids, discusses how existing theories can guide the interpretation of experimental data, and suggests future challenges for this field of research.

Water-Catalyzed Dehalogenation Reactions of the Isomer of CBr4 and Its Reaction Products and a Comparison to Analogous Reactions of the Isomers of Di- and Trihalomethanes

A combined experimental and theoretical study of the UV photolysis of a typical tetrahalomethane, CBr4, in water and acetonitrile/water was performed. Ultraviolet photolysis of low concentrations of CBr4 in water mostly leads to the production of four HBr leaving groups and CO2. Picosecond time-resolved resonance Raman (Ps-TR3) experiments and ab initio calculations indicate that water-catalyzed O-H insertion/HBr elimination of the isomer of CBr4 and subsequent reactions of its products lead to the formation of these products. The UV photolyses of di-, tri-, and tetrahalomethanes at low concentrations in water-solvated environments are compared to one another. This comparison enables a general reaction scheme to be deduced that can account for the different products produced by UV photolysis of low concentrations of di-, tri-, and tetrahalomethanes in water. The fate of the (halo)formaldehyde intermediate in the chemical reaction mechanism is the key to determining how many strong acid leaving groups are produced and which carbon atom final product is likely formed by UV photolysis of a polyhalomethane at low concentrations in a water-solvated environment.

Efficient dehalogenation of polyhalomethanes and production of strong acids in aqueous environments: Water-catalyzed O–H-insertion and HI-elimination reactions of isodiiodomethane (CH2I–I) with water

A combined experimental and theoretical study of the ultraviolet photolysis of CH2I2 in water is reported. Ultraviolet photolysis of low concentrations of CH2I2 in water was experimentally observed to lead to almost complete conversion into CH2(OH)2 and 2HI products. Picosecond time-resolved resonance Raman spectroscopy experiments in mixed water/acetonitrile solvents (25%-75% water) showed that appreciable amounts of isodiiodomethane (CH2I-I) were formed within several picoseconds and the decay of the CH2I-I species became substantially shorter with increasing water concentration, suggesting that CH2I-I may be reacting with water. Ab initio calculations demonstrate the CH2I-I species is able to react readily with water via a water-catalyzed O--H-insertion and HI-elimination reaction followed by its CH2I(OH) product undergoing a further water-catalyzed HI-elimination reaction to make a H2C=O product. These HI-elimination reactions produce the two HI leaving groups observed experimentally and the H2C=O product further reacts with water to produce the other final CH2(OH)2 product observed in the photochemistry experiments. These results suggest that CH2I-I is the species that reacts with water to produce the CH2(OH)2 and 2HI products seen in the photochemistry experiments. The present study demonstrates that ultraviolet photolysis of CH2I2 at low concentration leads to efficient dehalogenation and release of multiple strong acid (HI) leaving groups. Some possible ramifications for the decomposition of polyhalomethanes and halomethanols in aqueous environments as well as the photochemistry of polyhalomethanes in the natural environment are briefly discussed.

Direct observation of an isopolyhalomethane O–H insertion reaction with water: Picosecond time-resolved resonance Raman (ps-TR3) study of the isobromoform reaction with water to produce a CHBr2OH product

Picosecond time-resolved resonance Raman (ps-TR3) spectroscopy was used to obtain the first definitive spectroscopic observation of an isopolyhalomethane O-H insertion reaction with water. The ps-TR3 spectra show that isobromoform is produced within several picoseconds after photolysis of CHBr3 and then reacts on the hundreds of picosecond time scale with water to produce a CHBr2OH reaction product. Photolysis of low concentrations of bromoform in aqueous solution resulted in noticeable formation of HBr strong acid. Ab initio calculations show that isobromoform can react with water to produce a CHBr2(OH) O-H insertion reaction product and a HBr leaving group. This is consistent with both the ps-TR3 experiments that observe the reaction of isobromoform with water to form a CHBr2(OH) product and photolysis experiments that show HBr acid formation. We briefly discuss the implications of these results for the phase dependent behavior of polyhalomethane photochemistry in the gas phase versus water solvated environments.

Comparison of the dehalogenation of polyhalomethanes and production of strong acids in aqueous and salt (NaCl) water environments: Ultraviolet photolysis of CH[sub 2]I[sub 2]

The ultraviolet photolysis of CH(2)I(2) was studied in water and salt water solutions using photochemistry and picosecond time-resolved resonance Raman spectroscopy. Photolysis in both types of environments produces mainly CH(2)(OH)(2) and HI products. However, photolysis of CH(2)I(2) in salt water leads to the formation of different products/intermediates (CH(2)ICl and Cl(2) (-)) not observed in the absence of salt in aqueous solutions. The amount of CH(2)(OH)(2) and HI products appears to decrease after photolysis of CH(2)I(2) in salt water compared to pure water. We briefly discuss possible implications of these results for photolysis of CH(2)I(2) and other polyhalomethanes in sea water and other salt aqueous environments compared to nonsalt water solvated environments.