Department of Chemistry , University of California at Riverside , Riverside , California 92521 , United States.
Department of Chemistry and Biochemistry , California State University, Long Beach , Long Beach , California 90840 , United States.
J Phys Chem A. 2019 Nov 21;123(46):9957-9965. doi: 10.1021/acs.jpca.9b07797. Epub 2019 Nov 12.
The H atom product channels in the ultraviolet photodissociation of 2-propenyl (CHCCH) radical were investigated in the wavelength region 224-248 nm using photofragment translational spectroscopy. The CHCCH radicals were generated by 193 nm photodissociation of 2-chloropropene and 2-bromopropene precursors. The H atom photofragment yield spectra from both precursors revealed a broad feature peaking near 232 nm. The translational energy distributions of the H + CH products peaked around 7-8 kcal/mol and extended close to the maximum excess energy. The fraction of the total available energy released as products' translation was nearly a constant (∼0.16 using the 2-chloropropene precursor and ∼0.18 using the 2-bromopropene precursor) in the wavelength range 224-248 nm. The angular distribution of the H atom product was isotropic. Quasi-classical trajectory (QCT) calculations were performed on the ground-state potential energy surface of CHCCH for its decomposition at a 124 kcal/mol excitation energy (equivalent to 230 nm photolysis photon energy). The calculations yielded branching ratios for different dissociation product channels, 32% H + allene, 35% H + propyne, 0.5% H + cyclopropene, and 32% methyl + acetylene. The experimental and QCT translational energy distributions of the H atom loss channels qualitatively agreed, consistent with the main H atom product channels being the H + allene and H + propyne dissociations. The time scale of the 2-propenyl dissociation on the ground electronic state was calculated to be ∼2 ps, smaller compared to that of the overall UV photodissociation (≥10 ps, implied on the basis of the isotropic H atom product angular distribution). The mechanism of the UV photodissociation of 2-propenyl is consistent with unimolecular dissociation proceeding on the ground electronic state after internal conversion of the electronic excited states.
采用光碎片平移谱学技术，在 224-248nm 波长范围内研究了 2-丙烯基（CHCCH）自由基在紫外光解中的 H 原子产物通道。CHCCH 自由基是通过 193nm 光解 2-氯丙烯和 2-溴丙烯前体制备的。来自这两种前体的 H 原子光碎片产率谱在 232nm 附近呈现出一个宽的特征峰。H + CH 产物的平移能分布在 7-8kcal/mol 左右达到峰值，并接近最大过剩能。在 224-248nm 波长范围内，释放的总可用能量作为产物平移的分数几乎是一个常数（用 2-氯丙烯前体约为 0.16，用 2-溴丙烯前体约为 0.18）。H 原子产物的角分布是各向同性的。在 124kcal/mol 的激发能（相当于 230nm 光解光子能量）下，在 CHCCH 的基态势能表面上进行了准经典轨迹（QCT）计算，以模拟其分解。计算得到了不同分解产物通道的分支比，32%为 H + 丙烯，35%为 H + 丙炔，0.5%为 H + 环丙烯，32%为甲基+乙炔。实验和 QCT 平移能分布的 H 原子损失通道定性一致，与主要的 H 原子产物通道是 H + 丙烯和 H + 丙炔的分解一致。在基电子态上 2-丙烯的离解时间尺度计算为~2ps，与整体紫外光解（≥10ps，基于各向同性的 H 原子产物角分布推断）相比较小。2-丙烯的紫外光解机制与电子激发态内部转化后在基电子态上进行的单分子离解一致。
