Formation and Infrared Spectrum of the Open-Form 2-Bromoethyl Radical (2-C₂H₄Br^·) from Ultraviolet Irradiation of a C₂H₄/Br₂/ p-H₂Matrix

The addition reaction of halogens to alkenes is important in organic synthesis, but the reaction intermediate has rarely been detected. Whether the structure of the intermediate bromoethyl (C2H4Br·) radical is a bridged form or an open form is unclear. We took advantage of the diminished cage effect of solid p-H2 and employed infrared (IR) absorption to record the IR spectrum of C2H4Br· after photolysis of a C2H4/Br2/p-H2 matrix at 254 nm, followed by annealing. New spectral features at 676.9, 776.7, 1068.5, 1148.0, 3041.8, and 3126.8 cm-1 are assigned to the open-form 2-bromoethyl radical, according to their photolytic behavior and comparison with scaled harmonic vibrational wavenumbers and IR intensities calculated with the B2PLYPD3/6-311++G(2df,2p) method.