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.

中文翻译：

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C ₂ H ₄ / Br ₂ / p -H ₂基质的紫外线辐照形成开放式2-溴乙基自由基（2-C ₂ H ₄ Br ^•）的结构和红外光谱

卤素与烯烃的加成反应在有机合成中很重要，但是很少检测到反应中间体。中间体溴乙基（C ₂ H ₄ Br ^•）自由基的结构是桥接形式还是开放形式尚不清楚。我们利用了固态p -H _2的笼效应降低的优势，并利用红外（IR）吸收来记录C ₂ H ₄ / Br ₂ / p -H ₂光解后C ₂ H ₄ Br ^•的红外光谱在254nm处的基质，然后退火。根据开放性的2-溴乙基自由基的光解行为，并与通过B2PLYPD3计算出的比例谐波振动波数和IR强度进行比较，将在676.9、776.7、1068.5、1148.0、3041.8和3126.8 cm ^–1处的新光谱特征分配给了开放式2-溴乙基/ 6-311 ++ G（2df，2p）方法。