Spectroscopic Investigation of the Multiphoton Photolysis Reactions of Bromomethanes (CHBr3, CHBr2Cl, CHBrCl2, and CH2Br2) at Near-Ultraviolet Wavelengths (original) (raw)
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Journal of Physical Chemistry A, 1999
Reactions of methyl radicals with hydrogen bromide CH 3 + HBr f CH 4 + Br (1) and bromine atoms CH 3 + Br f CH 3 Br (2) were studied using excimer laser photolysis-transient UV spectroscopy at 297 ( 3 K over the 1-100 bar buffer gas (He) pressure range. Methyl radicals were produced by 193 nm (ArF) laser photolysis of acetone, (CH 3 ) 2 CO, and methyl bromide, CH 3 Br. Temporal profiles of methyl radicals were monitored by UV absorption at 216.51 nm (copper hollow cathode lamp with current boosting). The yield of acetyl radicals in photolysis of acetone at 193 nm was found to be less than 5% at 100 bar He based on the transient absorptions at 222.57 and 224.42 nm. The measured rate constants for reaction 1 are k 1 ) (2.9 ( 0.7) × 10 -12 , (3.8 ( 1.5) × 10 -12 , and (3.4 ( 1.3) × 10 -12 cm 3 molecule -1 s -1 at the buffer gas (He) pressures of 1.05, 11.2, and 101 bar, respectively. The rate data obtained in this study confirmed high values of the previous (low pressure) measurements and ruled out the possibility of interference of excited species. The measured rate constant is independent of pressure within the experimental error. The rate constant of reaction of methyl radicals with bromine atoms (2) was determined relative to the rate constant of methyl radical self-reaction, CH 3 + CH 3 f C 2 H 6 (3) in experiments with photolysis of CH 3 Br: k 2 /k 3 ) 0.92 ( 0.32, 1.15 ( 0.30, and 1.65 ( 0.26 at 1.05, 11.2, and 101 bar He, respectively. On the basis of the literature data for reaction 3, this yields k 2
The Journal of Physical Chemistry A, 2010
We report experimental and computational studies of the photolysis of atmospherically important 1,2dibromoethanes (1,2-C 2 X 4 Br 2 ; X) H, F) in Ar matrixes at 5 K. Using the pulsed deposition method, we find that significant conformational relaxation occurs for 1,2-C 2 H 4 Br 2 (EDB; observed anti/gauche ratio)30:1) but not for 1,2-C 2 F 4 Br 2 (TFEDB; anti/gauche) 3:1), which is traced to a larger barrier to rotation about the CC bond in the latter. Laser photolysis of matrix-isolated EDB at 220 nm reveals the growth of infrared bands assigned to the gauche conformer and C 2 H 4-Br 2 charge transfer complex (both as major products), and the C 2 H 4 Br radical and C 2 H 3 Br-HBr complex as minor (trace) products. The presence of the C 2 H 4-Br 2 complex is confirmed in the UV/visible spectrum, which shows an intense charge transfer band at 237 nm that grows in intensity upon annealing. In contrast to previous reports, our experimental and computational results do not support a bridged structure for the C 2 H 4 Br radical in either the gas phase or matrix environments. We also report on the laser photolysis of matrix-isolated TFEDB at 220 nm. Here, the dominant photoproducts are the anti and gauche conformers of the C 2 F 4 Br radical, the vibrational and electronic spectra of which are characterized here for the first time. The increase in yield of radical for TFEDB vs EDB is consistent with the stronger C-Br bond in the fluoro-substituted radical species. The photochemistry of the C 2 F 4 Br radical following excitation at 266 nm was investigated and found to lead C-Br bond cleavage and formation of C 2 F 4. The implications of this work for the atmospheric and condensed phase photochemistry of the alkyl halides is emphasized.
Laser Chemistry, 1996
Multiphoton dissociation processes of vinyl chloride and bromide were studied with a broadband ArF laser at 193 nm and with a narrowband tunable laser in the region of 212 nm, in the peak and in the threshold of the absorption band, respectively, for both compounds. Photolysis at 193 nm gives rise to the corresponding hydrogen halide in the B1∑+ excited state which results in an intense UV emission. However these emissions are absent when photodissociation was performed with the narrow band dye laser around 212 nm. These results, together with a calculation of the observed spectra, give further support to a mechanism which invokes one-photon resonant absorption of vibrationally hot ground state hydrogen halide as the process which populates the excited B1∑+ state of the fragment.
Photodissociation of bromobenzene in solution
Chemical Physics Letters, 2003
The photodissociation of bromobenzene in solution was investigated with ultrafast transient absorption spectroscopy following excitation at 266 nm. Ab initio calculations of lower singlet and triplet states were performed in order to guide the interpretations. The main feature of the kinetics measured between 300 and 930 nm in acetonitrile is a 9 AE 1 ps decay, which we mainly assign to predissociation. Similar decays were observed in hexane, dichloromethane and tetrachloromethane at 400 and 800 nm. Other features in acetonitrile, such as complicated short-time dynamics between 420 and 620 nm and a long-lived component, might indicate the involvement of lower triplet states.
The Journal of Chemical Physics, 2006
The A-X bands of the CH radical, produced in a 248 nm two-photon photolysis or in a supersonic jet discharge of CHBr 3 , have been observed via cavity ring-down absorption spectroscopy. Bromoform is a well-known photolytic source of CH radicals, though no quantitative measurement of the CH production efficiency has yet been reported. The aim of the present work is to quantify the CH production from both photolysis and discharge of CHBr 3 . In the case of photolysis, the range of pressure and laser fluences was carefully chosen to avoid postphotolysis reactions with the highly reactive CH radical. The CH production efficiency at 248 nm has been measured to be ⌽ = N͑CH͒ / N͑CHBr 3 ͒ = ͑5.0± 2.5͒10 −4 for a photolysis laser fluence of 44 mJ cm −2 per pulse corresponding to a two-photon process only. In addition, the internal energy distribution of CH͑X 2 ⌸͒ has been obtained, and thermalized population distributions have been simulated, leading to an average vibrational temperature T vib = 1800± 50 K and a rotational temperature T rot = 300± 20 K. An alternative technique for producing the CH radical has been tested using discharge-induced dissociation of CHBr 3 in a supersonic expansion. The CH product was analyzed using the same cavity ring-down spectroscopy setup. The production of CH by discharge appears to be as efficient as the photolysis technique and leads to rotationally relaxed radicals.
The Journal of Physical Chemistry A, 2006
Single photon dissociation of bromoform using synchrotron radiation has been investigated by Fourier transform visible fluorescence spectroscopy (FTVIS). The photodissociation of bromoform in the 12-18 eV energy range results in several products, among which are the CH(A 2 ∆) and CH(B 2 Σ) radicals. Vibrational and rotational state distributions of the CH(A 2 ∆) are determined from their fluorescence spectra. From the threshold photon energy above which emission from the CH(A 2 ∆) radicals is observed, the most likely process leading to CH(A) formation is CHBr 3 f CH + 3Br rather than CHBr 3 f CH + Br + Br 2. The rotational Boltzmann temperatures in the CH(A f X) emission spectra for V′) 0 and V′) 1 range between 1570 and 3650 K, depending on the excitation photon energy. From the high rotational excitation, the results suggest that the mechanism for the loss of three bromine atoms is most likely sequential. A small negative emission anisotropy of the CH(A) radicals [(I par-I per)/(I par + 2I per))-0.024 (0.005] is constant across the action spectrum; a small net absorption dipole of CHBr 3 in the vacuum ultraviolet is parallel to the 3-fold symmetry axis of the CHBr 3 molecule. The state distributions of the CH(A 2 ∆) radicals from multiphoton dissociation of bromoform using the 266 nm output (three photons) of a femtosecond laser (Boltzmann temperatures: T V′)0 rot) 4250 (300 K; T V′)1 rot) 3100 (550 K) are compared to those from the single photon dissociation results (Boltzmann temperatures: T V′)0 rot) 3650 (150 K; T V′)1 rot) 2400 (200 K) at the same total excitation energy under collision free conditions. The analysis of the CH(A) rotational populations shows hotter rotational populations for the femtosecond experiment, also suggesting sequential dissociation of the bromoform in the femtosecond experiment. The duration of the femtosecond laser pulse is approximately 180 fs, setting a limit on the time scales for the multiple dissociations.
Multiphoton ultraviolet photochemistry
Chemical Physics Letters, 1978
Multiphoton photodïssocïation bas been observed in C2N2, CaHa, CsH4, CHsOH, and CaHsOH. In aU of these mok cules fluorescente from small free radicak such as CN, CH, and OH bas been observed. None of the observcd fluorcscence can result from the absorption of a single 193 nm photon. Arguments are presented which suggest that the observed results are best explained by invoking a sequential absorption scheme where the excited molecule absorbs a second laser photon rather than predissociating. Jküssions have also been observed when HzO is photolyzed wïth the ArF laser. In Hz0 the evidence suggests that tIds enüssion arises from collisional dissociation of two excited Ha0 molecules.
Photochemistry of CH 2 BrCl: An ab Initio and Dynamical Study
The Journal of Physical Chemistry A, 2002
The photochemistry of CH 2 BrCl has been studied using laser-induced wave packet propagations on coupled multistate CASPT2 potentials calculated for the electronic ground and low-lying excited states, as a function of two reaction coordinates corresponding to the two halogen eliminations. The results indicate that the lowest excited state, with character 1 (nσ*), is strongly repulsive in both the C-Br and C-Cl directions. As a consequence, one-photon resonant excitation induces halogen fragmentation that occurs around 100 fs. The weaker C-Br bond dissociates directly, whereas the fragmentation of the stronger C-Cl bond involves a nσ*(C-Br)/nσ*(C-Cl) curve crossing pathway. The vertical spectrum for the resulting CH 2 Br and CH 2 Cl radicals has also been calculated, giving energies and oscillator strengths in close agreement with experimental UV spectral range and measured absorption cross-sections.
The Journal of Chemical Physics, 2013
Ultrafast deep-ultraviolet through near infrared (210-950 nm) transient absorption spectroscopy complemented by ab initio multiconfigurational calculations offers a global description of the photochemical reaction pathways of bromoform following 255-nm excitation in methylcyclohexane and acetonitrile solutions. Photoexcitation of CHBr 3 leads to the ground-state iso-CHBr 3 product in a large quantum yield (∼35%), formed through two different mechanisms: concerted excited-state isomerization and cage-induced isomerization through the recombination of the nascent radical pair. These two processes take place on different time scales of tens of femtoseconds and several picoseconds, respectively. The novel ultrafast direct isomerization pathway proposed herein is consistent with the occurrence of a conical intersection between the first excited singlet state of CHBr 3 and the ground electronic state of iso-CHBr 3. Complete active space self-consistent field calculations characterize this singularity in the vicinity of a second order saddle point on the ground state which connects the two isomer forms. For cage-induced isomerization, both the formation of the nascent radical pair and its subsequent collapse into ground-state iso-CHBr 3 are directly monitored through the deep-ultraviolet absorption signatures of the radical species. In both mechanisms, the optically active (i.e., those with largest Franck-Condon factors) C−Br−Br bending and Br−Br stretching modes of ground-state iso-CHBr 3 have the largest projection on the reaction coordinate, enabling us to trace the structural changes accompanying vibrational relaxation of the non-equilibrated isomers through transient absorption dynamics. The iso-CHBr 3 photoproduct is stable in methylcyclohexane, but undergoes either facile thermal isomerization to the parent CHBr 3 structure through a cyclic transition state stabilized by the polar acetonitrile medium (∼300-ps lifetime), and hydrolysis in the presence of water.
Photodissociation of bromocarbons at 193, 222, and 248 nm: Quantum yields of Br atom at 298 K
The Journal of Chemical Physics, 1992
The primary quantum yields, <Pa,, for the formation of Br atom in the photodissociation of CF, Br, and CH, Br at 248,222, and 193 nm, and of CF, Br at 222 and 193 nm were measured at 298 K. The bromine atoms were directly detected via resonance fluorescence following pulsed laser photolysis of the molecules of interest. The Br atom quantum yields in CF, Br, photolysis increased with decreasing wavelengths: 1.01 + 0.15, 1.63 + 0.19, and 1.96 + 0.27 at 248,222, and 193 nm, respectively. The a,, values in CH, Br and CF, Br were close to unity at all the wavelengths: 0.92 f 0.15 and 1.12 f 0.16 at 222 and 193 nm, respectively, for CF, Br; 1.01 + 0.16, 1.10 f 0.20, and 1.05 f 0.11 at 248, 222, and 193 nm, respectively, for CH,Br. Quantum yield of H atom formation in the photolysis of CH, Br at 193 nm was measured to be 0.002 f 0.001. H atom could not be detected in the photolysis at 248 and 222 nm. In all cases the @ar values were found to be independent of buffer gas pressure or the photolysis laser fluence. Our results suggest that the quantum yields for dissociation of all the molecules considered here are unity; therefore, atmospheric lifetime calculations carried out by assuming a unit dissociation quantum yield are correct. The nature of the photodissociation process is discussed in the light of previous and present results.