Matrix Infrared Spectrum and Bonding in the Dibromomethyl Radical

Abstract

Simultaneous condensation of bromoform and lithium atoms at high dilution in argon on a CsI window at 15°K produces new infrared absorptions which are assigned to lithium bromide and the dibromomethyl radical. The identity of dibromomethyl is confirmed by comparison of spectra obtained from HCBr₃, DCBr₃, and HCBr₂Cl precursors reacting with lithium and sodium. These new absorptions are assigned to the antisymmetric H–C–Br bending and C–Br stretching modes and the symmetric C–Br vibration. The antisymmetric vibrational assignments are supported by product‐rule and normal‐coordinate calculations which give the potential constants $F_{55}\text{\hspace{0.17em}=\hspace{0.17em}3.10\hspace{0.17em}±\hspace{0.17em}0.10}\mathrm{mdyn}\mathrm{/\AA }$ , $F_{56}\text{\hspace{0.17em}=\hspace{0.17em}0.32\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{mdyn}/\mathrm{rad}$ , and $F_{66}\text{\hspace{0.17em}=\hspace{0.17em}0.50\hspace{0.17em}±\hspace{0.17em}0.05}\mathrm{mdyn}\mathrm{·\AA /}{\mathrm{rad}}^2$ , while the symmetric C–Br mode yields an approximate force constant $F_{22}\text{\hspace{0.17em}=\hspace{0.17em}3.7\hspace{0.17em}±\hspace{0.17em}0.4}\mathrm{mdyn}\mathrm{/\AA }$ . The C–Br valence force constants for bromomethyl radicals exceed normal C–Br values, while electronic stabilization for these radicals is indicated by bond dissociation energies. Similar results for chloromethyl radicals, in contrast to fluoromethyl radicals, suggest that $\mathrm{(p–d)\pi }$ bonding between the free‐radical carbon orbital and the $d$ orbitals on chlorine and bromine might account for the stabilization of chloromethyl and bromomethyl radicals, which cannot occur for fluoromethyl radicals or molecules with completely satisfied valence.