Matrix Infrared Spectrum and Bonding in the Dichloromethyl Radical

Abstract

Simultaneous deposition of chloroform and lithium atoms at high dilution in argon on a CsI window at 15°K produces new infrared absorptions which are attributed to lithium chloride and the dichloromethyl radical. The identity of dichloromethyl is provided by comparison of spectra recorded after reactions of HCCl₃, DCCl₃, and HCCl₂Br with lithium and sodium and the detection of (HCCl₂)₂ following diffusion and the decrease in HCCl₂ absorptions. The new absorptions are assigned to the antisymmetric H–C–Cl bending and C–Cl stretching vibrations. These vibrational assignments are supported by product‐rule and normal‐coordinate calculations which give the potential constants $F_{55}\text{\hspace{0.17em}=\hspace{0.17em}3.60\hspace{0.17em}±\hspace{0.17em}0.10}\mathrm{mdyn}\mathrm{/\AA }$ , $F_{56}\text{\hspace{0.17em}=\hspace{0.17em}0.35\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{mdyn}/\mathrm{rad}$ , and $F_{66}\text{\hspace{0.17em}=\hspace{0.17em}0.56\hspace{0.17em}±\hspace{0.17em}0.05}\mathrm{mdyn}\mathrm{·\AA /}{\mathrm{rad}}^2$ . The carbon–chlorine valence force constant deduced here exceeds normal C–Cl values, while bond dissociation energies indicate that HCCl₂ is electronically stabilized. Similar results for bromomethyl 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 may account for the stabilization of chloromethyl and bromomethyl radicals, which cannot occur for fluoromethyl radicals or molecules with completely satisfied valence.