Transient resonance Raman spectroscopy and density functional theory investigation of iso-polyhalomethanes containing bromine and/or iodine atoms

Abstract

We report additional transient resonance Raman spectra and density functional theory computations for the products formed following ultraviolet photoexcitation of solution phase polyhalomethanes containing bromine and/or iodine atoms. We show that the iso-polyhalomethane photoproduct is responsible for the intense transient absorption band observed in the 350-470 nm region after ultraviolet excitation of polyhalomethanes in the solution phase. We examine the trends and correlation in the density functional theory optimized geometry and intense electronic absorption transition in the 350-470 nm region for the iso-polyhalomethanes containing bromine and/or iodine atoms. We explore the chemical reactivity of the iso-polyhalomethane species using density functional theory computations for the reaction of iso-CH 2 Br-Br with ethylene as an example. Our results and comparison with experimental data in the literature indicate that the iso-polyhalomethane species is most likely the methylene transfer agent in the cyclopropanation reactions of olefins using ultraviolet photoexcitation of polyhalomethanes in the solution phase. We briefly discuss the possibility that the photochemistry and chemistry of the iso-polyhalomethanes may give significant release of reactive halogens to the atmosphere.

Communications: Photoinitiated bond dissociation of bromoiodomethane in solution: Comparison of one-photon and two-photon excitations and the formation of iso-CH2Br–I and iso-CH2I–Br

“…4,5 Excitation at 193 nm favors C-Br cleavage ͑branching ratio of 3.5:1 C-Br:C-I͒. 5 The same trends are observed in solution phase experiments including steady state 6 and timeresolved resonance Raman [7][8][9] and femtosecond transient absorption at 266 nm. 10 Recent theoretical calculations provide additional insight into the dissociation of CH 2 BrI.…”

“…9,12 The calculations of Liu et al 12 suggest that the iso-CH 2 I -Br compound has a strong absorption band at ϳ315 nm ͑f = 0.46͒. The second transition at ϳ445 nm ͑f = 0.003͒ is significantly weaker than the corresponding transition of iso-CH 2 Br-I.…”

“…The complex is predicted to be more stable than iso-CH 2 Br-I by ϳ2.7 kcal/ mol in the most recent calculations 12 and ϳ4.1 kcal/ mol in earlier calculations. 7,9 Iso-CH 2 Br-I has been identified in nanosecond time-resolved resonance Raman measurements ͑vide infra͒. 7,9 In the work reported here we compare one-photon ͑ϳ266 nm͒ and two-photon ͑ϳ400 nm͒ excitations of bro- moiodomethane in 2-butanol.…”

“…7,9 Iso-CH 2 Br-I has been identified in nanosecond time-resolved resonance Raman measurements ͑vide infra͒. 7,9 In the work reported here we compare one-photon ͑ϳ266 nm͒ and two-photon ͑ϳ400 nm͒ excitations of bro- moiodomethane in 2-butanol. Pump-probe transient absorption measurements were performed using three different femtosecond Ti:sapphire laser systems with central wavelengths of 790-792 nm ͑ϳ60 fs, 700 J or 110 fs, 550 J͒ or 810 nm ͑ϳ60 fs, 400 J / pulse͒.…”

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

“…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][4][5][6][7][8][9][10][11][12][13][14][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.…”