Photochemistry of Iodoform in Methanol: Formation and Fate of the <i>Iso</i>‐CHI<sub>2</sub>‐I Photoproduct

El‐Khoury, Patrick Z.; Kwok, Wai Ming; Guan, Xiangguo; Ma, Chensheng; Phillips, David Lee; Tarnovsky, Alexander N.

doi:10.1002/cphc.200900122

Cited by 29 publications

(45 citation statements)

References 35 publications

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“…Compared with the UV-visible pump-probe experiments, these are more difficult to interpret, but are sensitive to structures that are blind to optical probes. This result was 346 S.A. Reid apparently in conflict with earlier time-resolved TA and Raman experiments [73,95]; however, a subsequent ultrafast TA study showed that iso-CHI 3 reacted readily with the solvent (methanol) to eliminate HI [83]. The absence of evidence for formation of the iso-CBr 4 species led these authors to conclude that isomer formation represented a minor channel.…”

Section: Sa Reidmentioning

confidence: 85%

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: Sa Reidmentioning

confidence: 85%

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

Reid

2014

International Reviews in Physical Chemistry

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“…4,[25][26][27] Cage-enhanced geminate recombination has significant yield to the iso products and these products have been observed in variety of different experiments. 3,25,26,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Ultraviolet excitation of CH 2 BrI yields iso-CH 2 Br-I or iso-CH 2 I-Br following C-I or C-Br bond cleavage, respectively. These products are predicted to have transitions in the visible or near UV with very large oscillator strength (∼0.5).…”

Section: Introductionmentioning

confidence: 99%

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

Anderson

Spears

Wilson

et al. 2013

The Journal of Chemical Physics

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It is well known that ultraviolet photoexcitation of halomethanes results in halogen-carbon bond cleavage. Each halogen-carbon bond has a dominant ultraviolet (UV) absorption that promotes an electron from a nonbonding halogen orbital (nX) to a carbon-halogen antibonding orbital (σ*C-X). UV absorption into specific transitions in the gas phase results primarily in selective cleavage of the corresponding carbon-halogen bond. In the present work, broadband ultrafast UV-visible transient absorption studies of CH2BrI reveal a more complex photochemistry in solution. Transient absorption spectra are reported spanning the range from 275 nm to 750 nm and 300 fs to 3 ns following excitation of CH2BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH2Br + I radical pairs, iso-CH2Br-I, and iso-CH2I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH2Br-I. Both iso-CH2Br-I and iso-CH2I-Br are observed in cyclohexane with a ratio of ~2.8:1. In acetonitrile this ratio is 7:1 or larger. The observation of formation of iso-CH2I-Br photoproduct as well as iso-CH2Br-I following 266 nm excitation is a novel result that suggests complexity in the dissociation mechanism. We also report a solvent and concentration dependent lifetime of iso-CH2Br-I. At low concentrations the lifetime is >4 ns in acetonitrile, 1.9 ns in 2-butanol and ~1.4 ns in cyclohexane. These lifetimes decrease with higher initial concentrations of CH2BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes.

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“…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. [40][41][42][43][44][45][46][47][48][49][50][51][52][53] For example, iso-CH 2 I 2 is the primary species responsible for the FIG. 1.…”

Section: Introductionmentioning

confidence: 99%

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

George

Kalume

Esselman

et al. 2011

The Journal of Chemical Physics

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Iso-polyhalomethanes are known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases. In this work, iso-bromoform (iso-CHBr(3)) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations, to further probe the structure, spectroscopy, and photochemistry of this important intermediate. Selected wavelength laser irradiation of CHBr(3) isolated in Ar or Ne matrices at ~5 K yielded iso-CHBr(3); the observed infrared and UV/visible absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CHBr(3) potential energy surface were characterized computationally using high-level methods in combination with correlation consistent basis sets. These calculations show that, while the corresponding minima lie ~200 kJ/mol above the global CHBr(3) minimum, the isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr(2) + Br asymptote. The photochemistry of iso-CHBr(3) was investigated by selected wavelength laser irradiation into the intense S(0) → S(3) transition, which resulted in back photoisomerization to CHBr(3). Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which lies energetically below the threshold of the radical channel. Subsequently, natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.

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Photochemistry of Iodoform in Methanol: Formation and Fate of the Iso‐CHI₂‐I Photoproduct

Cited by 29 publications

References 35 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

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

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

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Photochemistry of Iodoform in Methanol: Formation and Fate of the Iso‐CHI2‐I Photoproduct

Cited by 29 publications

References 35 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

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

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

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Photochemistry of Iodoform in Methanol: Formation and Fate of the Iso‐CHI₂‐I Photoproduct