Photoisomerization reaction of CH2BrI following A-band and B-band photoexcitation in the solution phase: Transient resonance Raman observation of the iso-CH2I–Br photoproduct

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 4.1 kcal/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. © 2000 American Institute of Physics.published_or_final_versio

Solvation and solvent effects on the short-time photodissociation dynamics of CH2I2 from resonance Raman spectroscopy

Resonance Raman spectra of CH2I2 have been obtained at excitation wavelengths of 369, 355, and 342 nm in cyclohexane solution and in methanol solution at excitation wavelengths of 355 and 342 nm. Resonance Raman spectra were also measured for CH2I2 in the vapor phase with an excitation wavelength of 355 nm. The resonance Raman spectra of CH2I2 exhibit most of their intensity in fundamentals, overtones, and combination bands of modes nominally assigned as the I–C–I symmetric stretch, the I–C–I bend, and the I–C–I antisymmetric stretch vibrations. The absorption spectra and resonance Raman intensities of the gas phase and methanol solution phase diiodomethane spectra were simulated using a simple model and time-dependent wave packet calculations. Normal mode coefficients from normal coordinate calculations were used to convert the motion of the wave packet on the excited electronic state surface from dimensionless normal coordinates into internal coordinates of the molecule. The short-time photodissociation dynamics of diiodomethane in the vapor phase shows that the two C–I bonds are lengthening by the same amount, the I–C–I angle becomes smaller, the H–C–I angles become larger, and the H–C–H angle becomes smaller.The two C–I bonds appear essentially equivalent in the Franck–Condon region of the gas phase photodissociation which implies that the molecule chooses which C–I bond is broken after the wave packet has left the Franck–Condon region of the potential energy surface. Comparison of the gas phase resonance Raman spectrum with solution phase spectra obtained in cyclohexane and methanol solvents reveals that the short-time photodissociation dynamics are noticeably changed by solvation with a large solvent-induced symmetry breaking observed. In the Franck–Condon region of the solution phase diiodomethane photodissociation in methanol solvent the two C–I bond become larger by differing amounts, the I–C–I angle becomes smaller, the H–C–H angle becomes smaller, and the H–C–I angles differ from the corresponding gas phase values. During the initial stages of the solution phase photodissociation (at least in methanol and cyclohexane solvents) the two C–I bonds are not the same and this suggests that the molecule chooses which C–I bond will be broken soon after photoexcitation. ©1996 American Institute of Physics.published_or_final_versio

Investigation of the short-time photodissociation dynamics of trans-1-bromo-2-iodoethane in the A-band absorption

We have obtained resonance Raman spectra and absolute Raman cross section measurements at five excitation wavelengths within the A-band absorption for 1-bromo-2-iodoethane in cyclohexane solution. The resonance Raman spectra have most of their intensity in the fundamentals, overtones, and combination bands of six Franck-Condon active vibrational modes; the nominal CCI bend, C-I stretch, C-Br stretch, C-C stretch, CH 2 wag with the Br atom attached to the CH 2 group, and CH 2 wag with the I atom attached to the CH 2 group. The resonance Raman intensities and A-band absorption spectrum were simulated using a simple model and time-dependent wave packet calculations. The simulation results and normal mode descriptions were used to find the short-time photodissociation dynamics in terms of internal coordinate displacements. The A-band short-time photodissociation dynamics for trans-1-bromo-2-iodoethane show that the C-I, C-Br, and C-C bonds as well as the CCI, CCBr, HCC, ICH, and BrCH angles have significant changes during the initial stages of the photodissociation reaction. This indicates the photodissociation reaction has a large degree of multidimensional character and suggests that the bromoethyl photofragment receives substantial internal excitation in so far as the short-time photodissociation dynamics determines the energy partitioning. Comparison of our results for 1-bromo-2-iodoethane with the A-band short-time dynamics of iodoethane, 1-chloro-2-iodoethane, and 1,2-diiodoethane and the trends observed for their A-band absorption spectra suggest that both the C-I and C-Br bonds experience a noticeable amount of photoexcitation. © 1999 American Institute of Physics.published_or_final_versio

Effect of geometrical conformation on the short-time photodissociation dynamics of 1-iodopropane in the A-band absorption

We have taken resonance Roman 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 C=α-carbon atom attached to the iodine atom, B=β-carbon atom attached to the α-carbon atom. G=methyl group carbon atom attached to the β-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. © 1998 American Institute of Physics.published_or_final_versio

Photoisomerization reaction of CH2BrI following A-band and B-band photoexcitation in the solution phase: Transient resonance Raman observation of the iso-CH2I–Br photoproduct

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 4.1 kcal/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. © 2000 American Institute of Physics.published_or_final_versio

Early-time photodissociation dynamics of chloroiodomethane in the A-band absorption from resonance Raman intensity analysis

We have obtained resonance Raman spectra and absolute Raman cross sections for h 2-chloroiodomethane (fourteen excitation wavelengths between 200 nm and 355 nm) and d 2-chloroiodomethane (for 282.4 nm excitation) in cyclohexane solution. Most of the intensity in the A-band resonance Raman spectra appears in the nominal C-I stretch overtones progression and combination bands of the nominal C-I stretch overtones with the fundamentals of the CH 2 wag, CH 2 scissor, and the Cl-C-I bend or C-Cl stretch fundamentals. The A-band absorption and absolute resonance Raman intensities were simulated using a simple model which included preresonant contributions to the fundamental Raman peaks and time-dependent wave packet calculations. The motion of the wave packet on the excited state surface was converted from dimensionless normal coordinates into internal coordinates using the results of normal coordinate calculations. The A-band short-time photodissociation dynamics of chloroiodomethane shows that the C-I bond lengthens, the I-C-Cl and H-C-I angles become smaller, and the H-C-Cl angles become larger. These internal coordinate motions which are associated with relatively low frequency modes are consistent with a simple impulsive "soft" radical model of the photodissociation and the CH 2Cl group changing to a more planar structure. However, the C-H bond length does not change much and the H-C-H angle (associated with higher frequency modes) becomes slightly smaller which is inconsistent with the "soft" radical model and the CH 2Cl group changing to a more planar structure. This suggests that an impulsive "semirigid" radical model may be more appropriate than the "soft" radical model to qualitatively describe the chloroiodomethane photodissociation. An ambiguity in the assignment of the 724 cm -1 Raman peak and its associated combination bands to combination bands of the nominal C-I stretch overtones with the fundamentals of the Cl-C-I bend or C-Cl stretch fundamentals limits what we are able to determine about the C-Cl bond length changes during the initial stages of the photodissociation. © 1996 American Institute of Physics.published_or_final_versio

Experimental study of the dynamics of D+H2 reactive and inelastic collisions below 1.0 eV relative energy

We report the results of state-to-state dynamics experiments on the D + H2 → HD + H reaction as well as D + H2 → H 2 † + D energy transfer at relative energies of 0.67 and 0.79 eV. Both product state distributions and absolute partial cross sections have been determined, from coherent anti-Stokes Raman scattering (CARS) spectra of the HD and H2 † products recorded under single-collision conditions following pulsed-laser photolysis of DI to generate the D atom reactant. At both energies and for both reactive and inelastic collisions there is a strong dynamical bias against rotational and vibrational excitation of the product. However, at 0.67 eV there is an enhancement of both the relative and absolute yield of HD (v′ = 1), and to a lesser extent H2 (v′ = 1), the only energetically accessible vibrationally excited product states. This may be the result of a Feshbach resonance at ≈ 0.65 eV, just above the v′ = 1 threshold energy. Product quantum state distributions from quasiclassical trajectory calculations are in fairly good agreement with the experimental results, except that they do not show the v′ = 1 enhancement at 0.67 eV. However, the partial cross sections from the trajectory calculations are systematically larger than those measured. © 1989 American Institute of Physics.published_or_final_versio

Resonance Raman study of the A-band short-time photodissociation dynamics of axial and equatorial conformers of iodocyclopentane

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 atom bending motions as well as the torsional motion about the α and β 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. © 1999 American Institute of Physics.published_or_final_versio

Transient resonance Raman spectroscopy and density functional theory investigation of iso-polyhalomethanes containing bromine and/or iodine atoms

Transient resonance Raman spectroscopy and density functional theory were used to investigate iso-polyhalomethanes containing bromine and/or iodine atoms. The iso-polyhalomethane species were found to have an intense electronic band in the range 350-470 nm. It was found that iso-polyhalomethane species is most likely the methylene transfer agent in the cyclopropanation reactions of olefins.published_or_final_versio

Time-resolved resonance Raman spectroscopy and density functional theory investigation of the CH2I-I isomer and CH2I2⋯I molecular complex products produced from ultraviolet photolysis of CH2I2 in the solution phase

The CH2I-I isomer and CH2I2···I molecular complex products produced from ultraviolet photolysis of CH2I2 in the solution phase was analyzed by using time-resolved resonance Raman spectroscopy. The structure and properties of the CH2I-I species and the CH2I2···I molecular complex and their reaction towards ethylene were compared. The results showed that the CH2I-I isomer reacts with ethylene to produce a cyclopropane product and I2 leaving group via a single step and low barrier to reaction.published_or_final_versio