Ultraviolet photodissociation of bromoform at 234 and 267 nm by means of ion velocity imaging

Xu, Duo; Francisco, Joseph S.; Huang, Jianhua; Jackson, William M.

doi:10.1063/1.1491877

Cited by 68 publications

(56 citation statements)

References 24 publications

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“…43,44 Finally, isomerization may also be important in the decomposition of primary radicals derived from halons. To illustrate, Figure 5 displays calculated (CCSDT//MP2/augcc-pVTZ) stationary points on the CHBr2 PES.…”

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“…Thus, while isomerization on the ground-state surface will lead to molecular products, the yield is relatively small, and, because the quantum yield for internal conversion following UV excitation is by no means unity, this mechanism cannot explain the 16−26% yield of Br2 reported in some previous photochemical studies. 43,44 Finally, isomerization may also be important in the decomposition of primary radicals derived from halons. To illustrate, Figure 5 displays calculated (CCSDT//MP2/augcc-pVTZ) stationary points on the CHBr2 PES.…”

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“…Excitation in the near-UV accesses n−σ* transitions that lead to rapid C−Br bond cleavage; 42 however, the formation of Br2 has also been reported, with quantum yields ranging from 0.16 at 266 nm and 0.26 at 237 nm. 43,44 This was suggested to involve internal conversion to the ground state and subsequent passage over a three-center TS leading to CHBr + Br2 products, calculated to lie 93.1 kcal/mol above the reactant. 44 However, North and coworkers have examined the photolysis of bromoform at 266 and 193 nm and conclude that Br2 is not a primary photoproduct, 45,46 which is supported by another recent experiment.…”

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Isomerization as a Key Path to Molecular Products in the Gas-Phase Decomposition of Halons

Kalume

George

Reid

2010

J. Phys. Chem. Lett.

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NOT THE PUBLISHED VERSION; this is the author's final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page. Letters, Vol. 1, No. 20 (2010): pg. 3090-3095. DOI. This article is © American Chemical Society and permission has been granted for this version to appear in e-Publications@Marquette. American Chemical Society does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from American Chemical Society. Journal of Physical Chemistry 2The decomposition of halons remains controversial concerning the branching between radical and molecular products. The latter channel, where it has been found, is presumed to occur via a constrained symmetric multicenter transition state. Isomerization pathways in the gas-phase chemistry of halons have rarely been considered, despite the fact that the iso-halons, which feature a halogen−halogen bond, are widely recognized as important reactive intermediates in condensed phases. In this Letter, detailed calculations and modeling of the unimolecular decomposition of several important halons, including CF2Cl2, CF2Br2, and CHBr3, reveal that isomerization is a key pathway to molecular products. This path is important for both halons and their primary radicals as the barrier to isomerization in these compounds is typically isoenergetic with the threshold for bond fission. Keywords: decomposition; halocarbons; iso-halomethanes; isomerization Halocarbons such as chlorofluorocarbons (CFCs) are famous for their role in ozone depletion, 1 and due to their past widespread industrial use, it is crucial to understand the pathways for their decomposition. Perhaps surprisingly, the decomposition of simple halons such as CF2Cl2, CF2Br2, and CHBr3 remains controversial concerning the branching between radical and molecular products. The latter channel, where it has been found, is usually assumed to involve a constrained symmetric multicenter transition state, which has not been identified computationally. 2 The iso-halons are well-known condensed-phase reactive intermediates that possess a halogen−halogen bond; 3-15 yet, few studies have suggested a role for these species in the gas-phase chemistry of halons. In recent studies of the multiphoton dissociation of the halons CHX3 and CX4, (X = Br,I), Quandt and coworkers suggested on the basis of secondary evidence a mechanism that involved the isospecies. 16,17 However, to date, conclusive evidence has not been provided for the role of isomerization in the thermal or photoinitiated decomposition of halons.Our interest in this topic began in recent studies of the spectroscopy and photochemistry of the weakly bound iso-CF2X2 (X = Br, I) species, which were trapped in Ar and Ne matrixes at 5 K. 18,19 Excitation into the intense near-UV band of iso-CF2Br2 resulted in back-isomerization to CF2Br2, 19 and intrinsic reaction coordinate (IRC) calculations showed that a first-order saddle point connects th...

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Isomerization as a Key Path to Molecular Products in the Gas-Phase Decomposition of Halons

Kalume

George

Reid

2010

J. Phys. Chem. Lett.

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“…Penzhorn [577], and Mack and Thrush [854]. Although the mechanism for O + H 2 CO has been considered to be the abstraction reaction yielding OH + HCO, Chang and Barker suggest that an additional channel yielding H + HCO 2 may be occurring to the extent of 30% of the total reaction.…”

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Environmental effects of large igneous province magmatism: a Siberian perspective

Black

Lamarque

Shields

et al. 2015

Volcanism and Global Environmental Change

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“…Xu and co-workers [17] reported the branching ratio of channels (5) and (6) ∆H o 298K (kcal mol -1 )…”

Section: Co(a A)-chemiluminescence Spectrummentioning

confidence: 99%

248-nm Laser Photolysis of CHBr₃/O-Atom Mixtures: Kinetic Evidence for UV CO(A) Chemiluminescence in the Reaction of Methylidyne Radicals with Atomic Oxygen

Vaghjiani

2005

J. Phys. Chem. A

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and other excited state products that formed in the photo-produced precursor + O-atom reactions were measured by recording their time-resolved chemiluminescence in discrete vibronic bands.The CO 4 th Positive transition (A 1 Π, v'=0 → X 1 Σ + , v"=2) near 165.7 nm was monitored in this work to deduce the pseudo-first-order decay kinetics of the CO(A)-chemiluminescence in the presence of various added substrates (CH 4 , NO, N 2 O, H 2 , and O 2 ). From this, the second-order rate coefficient values were determined for reactions of these substrates with the photo-produced precursors. The measured reactivity trends suggest that the prominent precursors responsible for the CO(A)-chemiluminescence are the methylidyne radicals, CH(X 2 Π) and CH(a 4 Σ -), whose Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 2 production requires the absorption of at least 2 laser photons by the photolysis mixture. The Oatom reactions with brominated precursors (CBr, CHBr and CBr 2 ), which also form in the photolysis, are shown to play a minor role in the production of the CO(A or a)-chemiluminescence. However, the CBr 2 + O-atom reaction was identified as a significant source for the 289.9-nm Br 2-chemiluminescence that was also observed in this work. The 282.2-nm OH-and the 336.2-nm NH-chemiluminescences were also monitored to deduce the kinetics of CH(X 2 Π) and CH(a 4 Σ -) reactions when excess O 2 and NO were present.

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Ultraviolet photodissociation of bromoform at 234 and 267 nm by means of ion velocity imaging

Cited by 68 publications

References 24 publications

Isomerization as a Key Path to Molecular Products in the Gas-Phase Decomposition of Halons

Isomerization as a Key Path to Molecular Products in the Gas-Phase Decomposition of Halons

Environmental effects of large igneous province magmatism: a Siberian perspective

248-nm Laser Photolysis of CHBr₃/O-Atom Mixtures: Kinetic Evidence for UV CO(A) Chemiluminescence in the Reaction of Methylidyne Radicals with Atomic Oxygen

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Ultraviolet photodissociation of bromoform at 234 and 267 nm by means of ion velocity imaging

Cited by 68 publications

References 24 publications

Isomerization as a Key Path to Molecular Products in the Gas-Phase Decomposition of Halons

Isomerization as a Key Path to Molecular Products in the Gas-Phase Decomposition of Halons

Environmental effects of large igneous province magmatism: a Siberian perspective

248-nm Laser Photolysis of CHBr3/O-Atom Mixtures: Kinetic Evidence for UV CO(A) Chemiluminescence in the Reaction of Methylidyne Radicals with Atomic Oxygen

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248-nm Laser Photolysis of CHBr₃/O-Atom Mixtures: Kinetic Evidence for UV CO(A) Chemiluminescence in the Reaction of Methylidyne Radicals with Atomic Oxygen