Time-resolved resonance Raman spectroscopy and density functional theory investigation of the CH2I–I isomer and CH2I2⋯I molecular complex products produced from ultraviolet photolysis of CH2I2 in the solution phase: Comparison of the structure and chemical reactivity of polyhalomethane isomers and polyhalomethane–halogen atom molecular complexes

Abstract

Time-resolved resonance Raman spectra are reported for different concentrations of CH2I2 in cyclohexane solution. The CH2I–I species is observed at low concentrations and it decays on the order of tens of nanoseconds to almost no signal at 100 ns and no other signal is observed up to 15 microseconds. Two species are observed at high concentrations. The first species CH2I–I spectra and lifetime are about the same as that found at low concentration of CH2I2 parent molecule and the second species is a CH2I2⋯I molecular complex observed on the nanosecond to microsecond time scale and formed from bimolecular reaction of iodine atoms with CH2I2 molecules. The chemical reactivity of the CH2I–I species and the CH2I2⋯I molecular complex towards carbon double bonds were investigated using density functional theory calculations. The structure and properties of the CH2I–I species and the CH2I2⋯I molecular complex and their reaction towards ethylene were compared. The CH2I–I species and the CH2I2⋯I both have weak I–I bonds that are the chromophores responsible for similar intense transient absorption bands. However, the geometry of the I–I bond relative to the C–I bond is noticeably different for these two species and this leads to distinctly different chemical reactivity toward carbon double bonds. The CH2I–I isomer readily reacts with ethylene to produce a cyclopropane product and I2 leaving group via a single step and low barrier to reaction while the CH2I2⋯I molecular complex reacts with ethylene to form an ethylene/I intermediate and a CH2I2 leaving group. Probable ramifications for other related molecule–halogen atom complexes are briefly discussed.