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

Abstract

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...

Roaming-mediated ultrafast isomerization of geminal tri-bromides in the gas and liquid phases

“…This isomerization pathway could be termed 'roamingmediated', because it involves the migration of the incipient bromine fragment and leads to the iso product species; however, this could not be confirmed without comparison to the same reaction in the gas phase. The isomeric form occupies a shallow minimum on the potential energy surface (PES)-somewhat below the C-Br bond radical dissociation asymptote [12][13][14][15] -and has been proposed 16 to be accessible on the way towards molecular bromine from photoexcited bromoform in the gas phase. This isomerization pathway in the gas phase can only occur through the internal rearrangement of atoms, that is, directly and without the involvement of any other species.…”

When isomerisation is electron transfer: the intriguing story of the iso-halocarbons

“…interest is the potential opening of the channel leading to carbene + dihalogen formation, which remains a topic of interest and controversy in gas-phase experiments up to the present day [33,34]. Molecular beam experiments carried out under single collision conditions show that the dominant process following UV excitation of these polyhalomethanes is cleavage of a carbon-halogen bond, yielding a radical pair [29,30,35,36].…”

“…For example, in the polyhalomethanes the isomerisation barrier lies much lower in energy than the tight three-centre TS leading to dihalogen elimination [33,41,96]. For example, in the polyhalomethanes the isomerisation barrier lies much lower in energy than the tight three-centre TS leading to dihalogen elimination [33,41,96].…”

Spectroscopic and computational studies of matrix-isolated iso-CHBr3: Structure, properties, and photochemistry of iso-bromoform

“…35,36 This finding is consistent with several other experimental and theoretical studies, 31,37,38 and our recent study of the importance of isomerization as a path to molecular products in halon decomposition has shown that an internal conversion mechanism involving isomerization is insufficient to explain the reported yields of Br 2 in the photolysis of bromoform at 248 nm. 39 The condensed phase photochemistry of bromoform and related halomethanes is slightly less controversial, Figure 1(b). Here, photolysis is known to lead to facile production of the iso-halomethanes following geminate recombination of the initially formed radical pair in the solvent cage, and the isomers have been shown to play a pivotal role in the reactivity of halomethanes in solution.…”