We examine the chemical reactions of the isodiiodomethane (CH2I-I), .CH2I and CH2I(+) species with ethylene using density functional theory computations. The CH2I-I species readily reacts with ethylene to give the cyclopropane product and an I2 product via a one-step reaction with a barrier height of approximately 2.9 kcal/mol. However, the.CH2I and CH2I(+) species have much more difficult pathways (with larger potential barriers) to react with ethylene via a two-step reaction mechanism. Comparison of experimental results to our present calculation results indicates that the CH2I-I photoproduct species is most likely the methylene transfer agent for the cyclopropanation reaction of olefins via ultraviolet photoexcitation of diiodomethane.
We report transient resonance Raman experiments that identify isodiiodomethane as the photoproduct
responsible for the ∼385 nm absorption band observed following ultraviolet excitation of diiodomethane in
liquid solutions. Comparison with previously reported gas-phase experiments and solution-phase resonance
Raman and femtosecond transient absorption results suggest that solvation leads to appreciable production of
the isodiiodomethane (H2C−I−I) photoproduct via the interaction of the initially formed CH2I and I fragments
with the solvent cage. The isodiiodomethane photoproduct is likely the species or an intermediate to the
species that reacts with alkenes in cyclopropanation reactions that use ultraviolet excitation of diiodomethane
in liquids.
We have done ab initio calculations to find the equilibrium geometries, rotational/inversion barriers, and
harmonic vibrational frequencies of several haloethyl radicals (XCH2CH2 and XCHCH3 where X = F, Cl,
Br). One equilibrium and two transition conformations for XCH2CH2 (X = Cl, Br) and XCHCH3 (X = F, Cl,
Br) were found on the calculated B3LYP/6-311++G(3df,3pd) potential energy surface. We discuss the effects
of the halo substituents on the haloethyl radicals investigated. The C−X bonds of the equilibrium β-haloethyl
radicals weaken due to hyperconjugative interaction enhancement as X goes from F to Br. The rotational
barriers about the C−C bond have been located using analytical methods. We have also made preliminary
vibrational assignments of two bromoethyl radical conformers to experimental transient resonance Raman
spectra obtained from the A-band photodissociation of 1-bromo-2-iodoethane.
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.
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