Concerted and sequential pathways of proton-coupled electron transfer in hydrogen halide elimination

Kalume, Aimable; George, Lisa; Cunningham, Nicole; Reid, Scott A.

doi:10.1016/j.cplett.2012.11.053

Cited by 7 publications

(18 citation statements)

References 48 publications

(49 reference statements)

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“…The energy scan looks very similar to those calculated for the iso-polyhalomethanes (compare Figure 13 with Figure 6, e.g. For example, the calculated PES of 1,1-dichloroethane (1,1-EDC) is very similar to that shown in Figure 14, with the exception that the barrier to HCl loss from the isomer is nearly coincident with that of back-isomerisation [148,149]. This suggested to us that a TS to proton abstraction and HBr elimination must lie close in energy, and such a TS was indeed found [148].…”

Section: Beyond the Polyhalomethanes: Studies Of Larger Systemssupporting

confidence: 72%

“…Calculations predict that iso-1,1-EDB is a minimum on the 1,1-EDB PES, and the calculated isomer structure bears much similarly to that of the related iso-dibromomethane. This suggested to us that a TS to proton abstraction and HBr elimination must lie close in energy, and such a TS was indeed found [148]. The corresponding weights for iso-CH 2 Br 2 calculated at the same level of theory are 49% (ion pair), 22% (ylide) and 20% (covalent).…”

Section: Beyond the Polyhalomethanes: Studies Of Larger Systemsmentioning

confidence: 72%

“…For example, at the M06-2X/aug-cc-pVTZ level, the calculated resonance weights are 50% for the ion-pair structure, 30% for the covalent structure and 12% for the ylide structure. These pathways are summarised in the stationary point diagram for the 1,1-EDB PES, shown in Figure 14 from calculations at the CCSD(T)//M06-2X/aug-cc-pVTZ level [148,149]. Despite this 360 S.A. Reid computational prediction of the stability of the isomer, we did not succeed in trapping iso-1,1,-EDB in rare gas matrices, instead observing HBr as the primary photoproduct.…”

Section: Beyond the Polyhalomethanes: Studies Of Larger Systemsmentioning

confidence: 89%

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

Reid

2014

International Reviews in Physical Chemistry

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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.

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Section: Beyond the Polyhalomethanes: Studies Of Larger Systemssupporting

confidence: 72%

Section: Beyond the Polyhalomethanes: Studies Of Larger Systemsmentioning

confidence: 72%

Section: Beyond the Polyhalomethanes: Studies Of Larger Systemsmentioning

confidence: 89%

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

Reid

2014

International Reviews in Physical Chemistry

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“…Motivated by our failure to trap iso-1,1,-EDB in rare gas matrixes and the observation instead of HBr as a significant photoproduct in steady-state experiments, we recently reported a theoretical study of halogen halide (HBr) elimination pathways on the ground state PES of 1,1-EDB. (39) This study clearly demonstrated the existence of two distinct mechanisms for HBr production, each involving coupled electron and proton transfer. A concerted proton-coupled electron transfer (PCET) pathway was found to be favored by ∼30 kJ/mol over a sequential electron transfer/proton transfer mechanism involving the iso-1,1-EDB as an intermediate.…”

Section: Introductionmentioning

confidence: 56%

Case of the Missing Isomer: Pathways for Molecular Elimination in the Photoinduced Decomposition of 1,1-Dibromoethane

et al. 2013

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We report an experimental and computational study of the photodecomposition pathways of a prototypical gem-dihalide, 1,1-dibromoethane (1,1-EDB), in the condensed phase. Following photolysis of the matrix isolated parent compound in Ar at 5 K, photoproducts are observed corresponding to Br2 elimination (+ C2H4 or C2H2) and HBr elimination (+ vinyl bromide). The elimination products are observed in the matrix as complexes. In contrast to our recent studies of the photolysis of matrix isolated polyhalomethanes, no evidence for the iso-1,1-EDB species is found, although studies of the matrix isolated 1,1-dibromo-2,2,2-trifluoroethane analogue show that the isomer is the dominant photoproduct. These results are examined in the light of theoretical studies that have characterized in detail the 1,1-EDB potential energy surface (PES). For Br2 elimination, a pathway from the isomer on the singlet PES is found which involves a simultaneous Br2 loss with 1,2-hydrogen shift; this pathway lies lower in energy than a concerted three-center elimination from the parent 1,1-EDB. For HBr elimination, our previous theoretical studies [Kalume, A.; George, L.; Cunningham, N.; Reid, S. A. Chem. Phys. Lett. 2013, 556, 35-38] have demonstrated the existence of concerted (single-step) and sequential pathways that involve coupled proton and electron transfer, with the sequential pathway involving the isomer as an intermediate. Here, more extensive computational results argue against a simple radical abstraction pathway for this process, and we compare experimental and computational results to prior results from the photolysis of the structural isomer, 1,2-EDB. These steady-state experiments set the stage for ultrafast studies of the dynamics of this system, which will be important in unraveling the complex photodecomposition pathways operative in condensed phases.

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“…First, a reduction of the ammonium cation in the presence of an activated carbon electrode is presumably linked to electronic transfer kinetics. Reactions in which electron and proton transfer is performed have been described either by two distinct steps, Electron Proton Transfer or Proton Electron Transfer (EPT or PET), or in the same concerted step, Concerted Proton Electron Transfer (CPET) [140][141][142]. Contrary to simple proton or electron transfer, CPET is more complicated and the coupling at the activated carbon /molecule (cation) interface influences the process both thermodynamically and kinetically.…”

Section: Parameters Influencing the Storage Mechanism According To Pilsmentioning

confidence: 98%

Room-Temperature Molten Salts: Protic Ionic Liquids and Deep Eutectic Solvents as Media for Electrochemical Application

Anouti

2015

Electrochemistry in Ionic Liquids

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Ionic liquids (ILs) are room-temperature molten salts (RTMS) composed mostly of organic ions that may undergo almost unlimited structural variations. ILs and deep eutectic solvents (DESs) have been applied in various fields, such as electrolytes for lithium ion batteries, redox flow batteries (RFBs), electrodeposition, and electropolishing, and even in fuel cells. This chapter covers the newest aspects of H-donor ILs in applications where their transport properties (ion conductivity, diffusion, and viscosity) are exploited, for example, as electrochemical solvents for energy storage where conventional media, organic solvents (in batteries), or water (in supercapacitors) fail. Metal nanoparticles' (NPs) formulation and stabilization in H-donor RTMS with some unique size, shape, and properties are also discussed. In these decidedly different materials, a mixture with a molecular solvent shows electroactivity that is not exhibited in more traditional systems, creating huge potential for energy storage and electrocatalysis. The remarkable recent increase in the level of interest in RTMS (protic ionic liquids, PILs, and DESs) in academics and industry is illustrated by the rapid growth in the number of publications on the topic (Fig. 7.1), with more than a 100-fold increase observed over the period 2004 to 2014.The first main objective of this chapter is to summarize the synthesis and physical properties of PILs and DESs and discuss their suitability as green solvents for different applications. The second main objective is to review the electrochemical applications of RTMS and discuss their advantages and disadvantages as electrolytes in comparison with an aprotic homologue for energy storage applications. I hope to generate interest in the wider community and encourage others to make use of proton-donor RTMS-PILs and DESs-in tackling scientific challenges.

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Concerted and sequential pathways of proton-coupled electron transfer in hydrogen halide elimination

Cited by 7 publications

References 48 publications

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

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

Case of the Missing Isomer: Pathways for Molecular Elimination in the Photoinduced Decomposition of 1,1-Dibromoethane

Room-Temperature Molten Salts: Protic Ionic Liquids and Deep Eutectic Solvents as Media for Electrochemical Application

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