Reactions of Alkali-Metal Atoms with Carbon Tetrabromide. Infrared Spectra and Bonding in the Tribromomethyl Radical and Dibromocarbene in Solid Argon

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Abstract
Simultaneous condensation at 15°K of beams of lithium atoms and carbon tetrabromide diluted in argon produces infrared absorptions which are identified as lithium bromide and the tribromomethyl radical. Comparisons within the series of radicals CCl3, CCl2Br, CClBr2, and CBr3 verify the molecular identity. Assignments to the carbon–bromine stretching vibrations are ν1 = 582 cm−1 and v3 = 773 cm−1, whereas the bending modes v2 and v4 were not observed. The stretching force constants Fr = 3.55 mdyn/Å and Frr = 0.70 mdyn/Å are compared with those of other bromocarbons and BBr3. The high carbon–bromine stretching-force constant can be rationalized by the fact that the tribromomethyl radical is resonance stabilized. Secondary reaction of lithium atoms with the tribromomethyl radical yields a sufficient concentration of dibromocarbene for infrared spectral study. Variation of the perbromochloromethane precursors and loss of CBr2 absorptions with attendant growth of the most intense C2Br4 absorption on sample warming furnish convincing evidence that dibromocarbene is isolated in the argon matrix. Assignments to the carbon–bromine stretching vibrations are v1 = 595.0 cm−1 and v3 = 640.5 cm−1, and the bending mode was not detected. The stretching-force constants Fr = 2.38 mdyn/Å and Frr = 0.18 mydn/Å indicate that C–Br single bonds are present in dibromocarbene, making it unlikely that the electron deficiency of the carbon atom is relieved by pi bonding with bromine.