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 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.

Resonance Raman spectra of 1,2-diiodoethane in cyclohexane solution have been obtained in the A-band absorption. A resonance Raman intensity analysis was done to examine the short-time photodissociation dynamics of 1, 2-diiodoethane. Significant bond length and bond angle changes were found for both C-I bonds and the C-C bond as well as the CCI and HCH bond angles. Comparison of the Franck–Condon region dynamics of 1,2-diiodoethane with diiodomethane suggests that the degree of coupling of the two C-I chromophores depends on both the relative geometry of the C-I bonds and their relative separation.

We have obtained Raman spectra for neat liquid iodocyclopentane and for two types of solid phases. Ab initio calculations were done to help elucidate the Raman band assignments to either the axial or equatorial conformations of iodocyclopentane. The liquid Raman spectrum displays bands for both axial and equatorial conformations of iodocyclopentane while the two solid phases exhibit a strong preference for either the axial conformation for the annealed solid phase or the equatorial conformation for the unannealed solid phase. We discuss the possible utility for using this phase-dependent conformation selection to examine the conformation dependence of solid-state photochemical reactions.

We have taken resonance Raman spectra and made absolute Raman cross section measurements at six excitation wavelengths for 1-iodopropane. The resonance Raman spectra have most of their Raman intensity in features that may be assigned as fundamentals, overtones, and combination bands of three Franck-Condon active vibrational modes the nominal C-I stretch, the nominal CCC bend, and the nominal CCI bend! for the trans and gauche conformations of 1-iodopropane. The resonance Raman and absorption cross sections of the trans and gauche conformations of 1-iodopropane were simulated using a simple model and time-dependent wave packet calculations. The results of the simulations were used in conjunction with the vibrational normal-mode coefficients to find the short-time photodissociation dynamics of trans and gauche conformers of 1-iodopropane in terms of internal coordinate changes. The trans and gauche conformers display significantly different Franck-Condon region photodissociation dynamics, which indicates that the C-I bond breaking is conformational dependent. In particular, there are large differences in the trans and gauche short-time photodissociation dynamics for the torsional motion (xGBx) about the GB carbon–carbon bond and the GBC angle where C5a-carbon atom attached to the iodine atom, B5b-carbon atom attached to the a-carbon atom, G5methyl group carbon atom attached to the b-carbon atom!. The major differences in the trans and gauche A-band short-time photodissociation dynamics can be mostly explained by the position of the C-I bond in the trans and gauche conformers relative to the plane of the three carbon atoms of the n-propyl group of 1-iodopropane.