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

Abstract

This review examines recent studies of the spectroscopy, dynamics, reactivity and electronic structure of iso-halocarbons, which are isomers of organic halogens that are critically important reactive intermediates in the chemistry of these species. We approach this review from the viewpoint of isomerisation as electron transfer, in as much as the dominant resonance structure of the isomer is an ion pair of halocarbenium ion and halide anion. We show that this perspective is important in understanding the spectroscopy, dynamics and chemical reactivity of the iso-halocarbons. We examine experimental methods for study of the isomer, focusing on the complementary nature of steady state matrix isolation experiments and time-resolved studies, typically carried out in solution. We review experimental data concerning the solvent dependence of the isomer lifetime in solution, examine S N 1 style nucleophilic reactions of the isomer with water and related solvents, discuss the role of the isomer as methylene transfer agent in cyclopropanation reactions and probe the possible role of the isomer in photoinduced halogen exchange. We then discuss the role of the isomer in the gas-phase chemistry and photochemistry of halocarbons, where isomerisation is demonstrated as a path to molecular elimination. Moving beyond the polyhalomethanes, we describe the existence of proton-coupled electron-transfer pathways involving the isomer in the gem-dihaloethanes, examine the potential importance of spin-orbit coupling and the role of triplet states and briefly discuss the potential importance of similar isomeric structures in other organic molecules. We conclude with future perspectives and research on this intriguing class of intermediates.

Transient isomers in the photodissociation of bromoiodomethane

“…The experimental lifetime is comparable with the aforementioned experiments of CH 2 IBr in polar solvents. In analogy with CH 2 I-I, the more rapid decay of CH 2 Br-I might be caused by a destabilization and dissociation of the Br-I bond through a specific solvation and electron transfer, 40,58 such as CH 2 Br-I −→ CH 2 Br + + I . Nevertheless, we did not include this pathway in our kinetic model because the kinetic fitting did not improve with respect to a more simple one.…”

“…The photochemistry of organo-halides 40 is of strong interest in atmospheric chemistry due to their ozone depletion ability in the stratosphere. [41][42][43] In particular, dihalomethanes have been recognized as potentially important since they are naturally emitted into the atmosphere as bio-products of several terrestrial and marine living beings [44][45][46][47][48][49] with an apparently puzzling chemistry.…”

VUV Photofragmentation of Chloroiodomethane: The Iso-CH2I–Cl and Iso-CH2Cl–I Radical Cation Formation

“… 30 , 31 It has been reported that condensed-phase photolysis of XCH 2 Y geminal species induces a halogen shift leading to iso-halomethane isomers where a halogen–halogen bond is formed (CH 2 X-Y). 32 , 33 Reid et al 34 , 35 demonstrated that this isomerization in the gas phase is a pathway to molecular products (XY) comparable to the simple C-halo bond fission. Other works demonstrate that iso-dihalomethanes are reactive photointermediates in solution leading to the cyclopropanation of alkenes.…”

“…In this work we focused on the VUV photofragmentation of chloroiodomethane, ClCH 2 I, a VOIC present in the environment due to marine microalgae and volcanic activity . Its photochemistry is a source of iodine atoms which are further involved in several chemical physics processes. , It has been reported that condensed-phase photolysis of XCH 2 Y geminal species induces a halogen shift leading to iso-halomethane isomers where a halogen–halogen bond is formed (CH 2 X-Y). , Reid et al , demonstrated that this isomerization in the gas phase is a pathway to molecular products (XY) comparable to the simple C-halo bond fission. Other works demonstrate that iso-dihalomethanes are reactive photointermediates in solution leading to the cyclopropanation of alkenes. − The neutral ClCH 2 I molecule has been extensively studied, − but to the best of our knowledge, less information exists for the radical ion.…”

Competing Molecular and Radical Pathways in the Dissociation of Halons via Direct Chemical Dynamics Simulations

“…35 Electronic structure theory has provided insights into the structure and energetics of isohalons but their role in the gas phase chemistry of halons is not well established. 38 Kalume et al, 37 has reported detailed electronic structure calculations (at the CCSD(T)//MP2/augcc-pVTZ level of theory) characterizing the radical and molecular dissociation pathways of CF 2 Cl 2 , CF 2 Br 2 , and CHBr 3 . In their calculations, they identified nearly isoenergetic pathways for radical formation and isomerization for all three species.…”