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

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 ${\nu }_1\text{\hspace{0.17em}=\hspace{0.17em}582}{\mathrm{cm}}^{\text{−1}}$ and $v_3\text{\hspace{0.17em}=\hspace{0.17em}773}{\mathrm{cm}}^{\text{−1}}$ , whereas the bending modes $v_2$ and $v_4$ were not observed. The stretching force constants $F_r\text{\hspace{0.17em}=\hspace{0.17em}3.55 mdyn/Å}$ and $F_{\mathrm{rr}}\text{\hspace{0.17em}=\hspace{0.17em}0.70 mdyn/Å}$ 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 $v_1\text{\hspace{0.17em}=\hspace{0.17em}595.0}{\mathrm{cm}}^{\text{−1}}$ and $v_3\text{\hspace{0.17em}=\hspace{0.17em}640.5}{\mathrm{cm}}^{\text{−1}}$ , and the bending mode was not detected. The stretching‐force constants $F_r\text{\hspace{0.17em}=\hspace{0.17em}2.38}\mathrm{mdyn}/\mathrm{\AA }$ and $F_{\mathrm{rr}}\text{\hspace{0.17em}=\hspace{0.17em}0.18}\mathrm{mydn}/\mathrm{\AA }$ 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.