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 excitation of polyhalomethanes in condensed phase environments.

We report a preliminary picosecond Stokes and anti-Stokes time-resolved resonance Raman ～267nm pump and 400nm probe excitation wavelengths! investigation of the initial formation and vibrational cooling of the iso-CH2I-I photoproduct species produced after ultraviolet excitation of diiodomethane in room temperature solutions. A comparison of the picosecond resonance Raman spectra with previously reported nanosecond transient resonance Raman spectra and density functional theory computations shows that the iso-CH2I–I photoproduct species is predominantly responsible for the ;385 nm transient absorption band observed from several picoseconds to nanoseconds after ultraviolet excitation of diiodomethane in the solution phase. Similar results were obtained in both nonpolar solution ~cyclohexane solvent! and polar solution~ acetonitrile! solvent. The picosecond resonance Raman spectra confirm that the iso-CH2I-I photoproduct species is formed vibrationally hot within several picoseconds and then subsequently undergoes vibrational cooling on the 4-50ps time scale. This is consistent with the absorption bands changes occurring over similar times in a recent femtosecond transient absorption study. We discuss a possible qualitative scenario for the formation of the iso-CH2I-I species that is in agreement with the available gas phase experimental results for the ultraviolet photodissociation reaction of diiodomethane and gas phase collisional deactivation studies of the CH2I radical. The proposed hypothesis is consistent with the lack of distinct resonance Raman bands in the first few picoseconds of our solution phase spectra of the iso-CH2I-I photoproduct as well as previously reported femtosecond transient absorption bands that are broad and weak in the 300-500nm region over the 0.3-3ps time scale.

We report additional transient resonance Raman spectra and density functional theory computations for the products formed following ultraviolet photoexcitation of solution phase polyhalomethanes containing bromine and/or iodine atoms. We show that the iso-polyhalomethane photoproduct is responsible for the intense transient absorption band observed in the 350-470nm region after ultraviolet excitation of pnlyhalomethanes in the solutinn phase. We examine the trends and correlation in the density functional theory optimized geometry and intense electronic absorption transition in the 350-470 nm region for the iso-polyhalomethanes containing bromine and/or iodine atoms. We explore the chemical reactivity of the iso-polyhalomethane species using density functional theory computations for the reaction of iso-CH2-Br-Br with ethylene as an example. Our results and comparison with experimental data in the literature indicate that the iso-polyhalomethane species is most likely the methylene transfer agent in the cyclopropanation reactions of olefins using nltraviolet photoexcitation of polyhalomethanes in the solution phase. We briefly discuss the possibilitv that the photochemistry and chemistry of the iso-polyhalomethanes may give significant release of reactive halogens to the atmosphere.

We present nanosecond transient resonance Raman experiments that investigate the photoproduct species formed following A-band and B-band excitation of bromoiodomethane in room temperature cyclohexane solutions. Density functional theory calculations were also performed for several species that have been proposed as photoproducts for photodissociation of bromoiodomethane in the condensed phase. Comparison of the experimental resonance Raman spectra to density functional theory computational results and results for the closely related iso-CH2I-I and iso-CH2Br-Br species demonstrated that the iso-CH2I–Br species is mainly responsible for a transient absorption spectrum that appears after either A-band or B-band photoexcitation of bromoiodomethane in cyclohexane solution. This is in contrast to previous results for low temperature (12 K) solids where mainly the iso-CH2Br-I species was observed following A-band photoexcitation of bromoiodomethane. Further density functional theory computational results indicate that the iso CH2I–Br species is noticeably more stable than the iso-CH2Br-I species by about 4.1kcal/mol. This suggests that although both iso-CH2I–Br and iso-CH2Br–I species may be initially produced following ultraviolet excitation of bromoiodomethane in cyclohexane solution, only the more stable isomer has a sufficiently long lifetime to be observed in our nanosecond time-scale transient resonance Raman experiments. We compare results for the bromoiodomethane ultraviolet photodissociation/photoisomerization reactions in the condensed phase to those of the closely related diiodomethane system and discuss a probable mechanism for the formation of the iso-bromoiodomethane species in the condensed phase.

We have obtained resonance Raman spectra of iodocyclopentane in cyclohexane solution at three excitation wavelengths resonant with the A-band absorption. The A-band resonance Raman spectral bands can be assigned to fundamentals, overtones, and combination bands of seven axial conformer and eight equatorial conformer Franck–Condon active modes. The resonance Raman and absorption cross sections were simultaneously simulated using wave packet calculations and a simple model. The best fit parameters of the simulations and the normal mode descriptions were used to determine the A-band short-time photodissociation dynamics of the axial and equatorial conformers of iodocyclopentane. The axial and equatorial conformers exhibit noticeably different short-time photodissociation dynamics that suggest that the C-I bond cleavage process is conformation dependent. The axial conformer short-time photodissociation dynamics have larger changes in the carbon-carbon stretch and three carbon atombending motions as well as the torsional motion about the a and b carbon atom bond. The CCI bending motions for the axial and equatorial conformers of iodocyclopentane as well as previously reported results for the equatorial conformer of iodocyclohexane are significantly smaller than CCI bending motions found for most noncyclic iodoalkanes examined so far. This suggests that the cyclic backbone restricts the initial motion of the C-I bond cleavage along the CCI bend in iodocycloalkanes compared to the noncyclic iodoalkanes.