Yun-ang Li

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.

Transient resonance Raman spectroscopy and density functional theory investigation of iso-CHBr2Cl and iso-CCl3Br photoproducts produced following ultraviolet excitation of CHBr2Cl and CCl3Br

We report transient resonance Raman spectra and density functional theory calculations for the photoproducts produced after ultraviolet excitation of CHBr2Cl and CCl3Br in cyclohexane solution. Comparison of the computed vibrational frequencies to the experimental Raman frequencies revealed that the iso-CHBrCl–Br and iso-CHClBr–Br species are mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CHBr2Cl. Similar comparisons for CCl3Br showed the iso-CCl2Cl–Br species is mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CCl3Br. Additional density functional theory computations were done to examine the chemical reactions of iso-CH2Br–Cl and iso-CH2Cl–Br with ethylene to give cyclopropane and Br–Cl product. We briefly discuss the possibility for release of reactive halogens into the atmosphere via the photochemical and chemical reactions of iso-polyhalomethane molecules formed after ultraviolet excit...

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.

Observation of a HI leaving group following ultraviolet photolysis of CH2I2 in water and an ab initio investigation of the O–H insertion/HI elimination reactions of the CH2I–I isopolyhalomethane species with H2O and 2H2O

We report an experimental observation of H+ and I− being formed following ultraviolet photolysis of CH2I2 in liquid water. The concentration of H+ and I− are about the same and suggest a reaction that produces a HI leaving group. A preliminary ab initio investigation is reported for the O–H insertion reactions of the isodiiodomethane (CH2I–I) carbenoid species with H2O and 2H2O and these are compared to the related reactions for the dichlorocarbene (:CCl2) species. The CH2I–I carbenoid reacts with water via an O–H insertion/HI elimination reaction mechanism. These reactions produce a HI leaving group whereas the dichlorocarbene O–H insertion reaction has no need for a leaving group. The differing structures and properties of the CH2I–I and the :CCl2 species can account for the differences in their O–H insertion reactions. The CH2I–I species O–H insertion/HI elimination reactions can be catalyzed by a second water molecule when two water molecules are available and this occurs in a way similar to that prev...

Comparison of the dehalogenation of polyhalomethanes and production of strong acids in aqueous and salt (NaCl) water environments: Ultraviolet photolysis of CH2I2

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.

Resonance Raman investigation of the short-time dynamics of the ultraviolet photodissociation of bromoform

We report ultraviolet resonance Raman spectra of bromoform (CHBr3) in cyclohexane solution. The resonance Raman spectra show significant intensity in the overtones of the nominal Br-C-Br symmetric bend (v 6), the nominal H-C-Br asymmetric bend (v3), the nominal Br-C-Br symmetric stretch (v 2) and the nominal Br-C-Br asymmetric stretch (v 5) vibrational modes suggesting that the short-time photodissociation dynamics have noticeable multidimensional character. The lack of strong combination bands between several of the Franck-Condon active modes suggests that more than one electronic transition contribute to the resonance Raman spectra. We briefly discuss the ultraviolet short-time photodissociation dynamics of bromoform and the potential implications for the secondary photodissociation reactions of the initially formed CHBr2 radical.

Transient resonance Raman and density functional theory investigation of CH3I–I produced from ultraviolet photolysis of iodomethane in the solution phase

Abstract Transient resonance Raman spectra are reported for the CH 3 I–I species formed after ultraviolet photolysis of iodomethane in cyclohexane and acetonitrile solvents. The spectra display most of their Raman intensity in the overtone progression of the nominal I–I stretch mode and the C–I stretch fundamental. Density functional theory calculations were done for the CH 3 I–I species to help better understand its properties. We compare our results for the CH 3 I–I species to those previously reported for CH 2 I–I and find that the C–I–I geometry significantly influences the perturbation of the C–I bond and the chemical reactivity of the complex species.

Transient resonance Raman investigation of CHBr2 and CHBrCl radicals

Abstract Transient resonance Raman spectra of CHBr 2 and CHBrCl radicals following ultraviolet photolysis of CHBr 3 and CHBr 2 Cl, respectively, in methanol solution are reported. Most of the Raman intensity appears in the C–Br stretch and Br–C–Br bend or Cl–C–Br bend fundamentals accompanied by moderate intensity in their overtones and/or combination bands. The CH 2 Br radical was previously observed to have only one predominant Franck–Condon active mode (the C–Br stretch) with a strong overtone progression. The Franck–Condon region photodissociation dynamics of the CHBr 2 and CHBrCl radicals appear to have more multidimensional character compared to halomethyl radicals like CH 2 Br.

Measuring the carrier dynamics of photocatalyst micrograins using the Christiansen effect

The optical measurement of photocatalyst materials is subject to Mie scattering when the particle size is comparable to the wavelength of the probe light. A novel approach was developed to deal with this scattering problem in the transient spectroscopy of photocatalyst micrograins using the Christiansen effect because the probe light in the vicinity of the Christiansen frequency can be transmitted. Scattering theory was used to analyze the transient spectra of micrograins and estimate the extinction coefficient at the Christiansen frequency. The Drude-Lorentz model was used to calculate the complex refractive index considering the contributions from both phonons and free carriers. We found that the net photogenerated carrier density was linearly correlated with the absorbance at the Christiansen frequency. With the parameters obtained from Raman scattering measurements, the absolute net carrier density was also determined. We further demonstrated the versatility of this method by applying it to the photogenerated carrier dynamics of intrinsic 6H-SiC grains. The transient broadband mid-IR spectra were measured by the pump-probe technique, and the transient absolute net carrier density was estimated. The carrier relaxation dynamics was fitted with three components with lifetime constants that agreed well with those obtained for SiC by transient broadband THz conductivity spectroscopy. We predict that this method could be extended to other photocatalytic materials with suitable probe frequencies.

A Density Functional Theory Study of the Reactions of Dichlorocarbene and Isodichloromethane with H2O

A density functional theory investigation of the reactions of dichlorocarbene and isodichlorocarbene with H2O is reported. The reactions are found to be noticeably different for dichlorocarbene and isodichloromethane and can be attributed to their different structures and properties. The isodichloromethane species can react with H2O by both direct and indirect O–H insertion reactions with the indirect reaction having a much lower barrier to reaction than the direct reaction. However, the dichlorocarbene reacts with H2O predominantly via a direct OH insertion reaction that has a barrier to reaction noticeably lower than both of the isodichloromethane reactions.