Kao Min-Hsien, Orr-Ewing Andrew J
School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.
J Phys Chem A. 2022 Oct 6;126(39):6934-6943. doi: 10.1021/acs.jpca.2c05327. Epub 2022 Sep 22.
Although ultraviolet (UV)-induced photochemical cleavage of carbon-halogen bonds in gaseous halocarbons is mostly homolytic, the photolysis of chlorobenzene in solution has been proposed to produce a phenyl cation, c-CH, which is a highly reactive intermediate of potential use in chemical synthesis and N activation. Any evidence for such a route to phenyl cations is indirect, with uncertainty remaining about the possible mechanism. Here, ultrafast transient absorption spectroscopy of UV-excited (λ = 240 and 270 nm) chlorobenzene solutions in fluorinated (perfluorohexane) and protic (ethanol and 2,2,2-trifluoroethanol) solvents reveals a broad electronic absorption band centered at 540 nm that is assigned to an isomer of chlorobenzene with both charge-separated and triplet-spin carbene character. This spectroscopic feature is weaker, or absent, when experiments are conducted in cyclohexane. The intermediate isomer of chlorobenzene has a solvent-dependent lifetime of 30-110 ps, determined by reaction with the solvent or quenching to a lower-lying singlet state. Evidence is presented for dissociation to -benzyne, but the intermediate could also be a precursor to phenyl cation formation.
虽然紫外线（UV）诱导气态卤代烃中碳 - 卤键的光化学裂解大多是均裂的，但有人提出氯苯在溶液中的光解会产生苯基阳离子（c-CH），它是一种在化学合成和氮活化中具有潜在用途的高活性中间体。关于这种生成苯基阳离子途径的任何证据都是间接的，其可能的机制仍存在不确定性。在此，对在氟化（全氟己烷）和质子性（乙醇和2,2,2 - 三氟乙醇）溶剂中紫外线激发（λ = 240和270 nm）的氯苯溶液进行的超快瞬态吸收光谱研究揭示了一个以540 nm为中心的宽电子吸收带，该吸收带被归因于具有电荷分离和三重态自旋卡宾特征的氯苯异构体。当在环己烷中进行实验时，这种光谱特征较弱或不存在。氯苯的中间异构体具有30 - 110 ps的溶剂依赖性寿命，这是通过与溶剂反应或猝灭到较低能级的单重态来确定的。文中给出了其解离为 - 苯炔的证据，但该中间体也可能是苯基阳离子形成的前体。
