Formation and Infrared Spectrum of the Open-Form 2-Bromoethyl Radical (2-C2H4Br•) from Ultraviolet Irradiation of a C2H4/Br2/p-H2 Matrix.

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.