Asmus Klaus-Dieter, Hug Gordon L, Bobrowski Krzysztof, Mulazzani Quinto G, Marciniak Bronislaw
Faculty of Chemistry, Adam Mickiewicz University, 60-780 Poznan, Poland.
J Phys Chem A. 2006 Jul 27;110(29):9292-300. doi: 10.1021/jp061944z.
The ()OH-induced oxidation of 1,3,5-trithiacyclohexane (1) in aqueous solution was studied by means of pulse radiolysis with optical and conductivity detection. This oxidation leads, via a short-lived ()OH radical adduct (<1 micros), to the radical cation 1(+) showing a broad absorption with lambda(max) equal to 610 nm. A defined pathway of the decay of 1(+) is proton elimination. It occurs with k = (2.2 +/- 0.2) x 10(4) s(-1) and yields the cyclic C-centered radical 1(-H)(). The latter radical decays via ring opening (beta-scission) with an estimated rate constant of about 10(5) s(-1). A distinct, immediate product (formed with the same rate constant) is characterized by a narrow absorption band with lambda(max) = 310 nm and is attributed to the presence of a dithioester function. The formation of the 310 nm absorption can be suppressed in the presence of oxygen, the rationale for this being a reaction of the C-centered cyclic radical 1(-H)() with O(2). The disappearance of the 310 nm band (with a rate constant of 900 s(-1)) is associated with the hydrolysis of the dithioester functionality. A further aspect of this study deals with the reaction of H() atoms with 1 which yields a strongly absorbing, three-electron-bonded 2sigma/1sigma radical cation 1(S therefore S)-H (lambda(max) = 400 nm). Its formation is based on an addition of H() to one of the sulfur atoms, followed by beta-scission, intramolecular sulfur-sulfur coupling (constituting a ring contraction), and further stabilization of the S therefore S bond thus formed by protonation. 1(S therefore S)-H decays with a first-order rate constant of about 10(4) s(-1). Its formation can be suppressed by the addition of oxygen which scavenges the H() atoms prior to their reaction with 1. Complementary time-resolved conductivity experiments have provided information on the quantification of the 1(*+) radical cation yield, the cationic longer-lived follow-up species, extinction coefficients, and kinetics concerning deprotonation processes as well as further reaction steps after hydrolysis of the transient dithioesters. The results are also discussed in the light of previous photochemical studies.
通过脉冲辐解结合光学和电导率检测手段，研究了水溶液中（）OH引发的1,3,5 - 三硫杂环己烷（1）的氧化反应。该氧化反应通过一个短寿命的（）OH自由基加合物（<1微秒），生成自由基阳离子1（* +），其在λ（max）等于610 nm处呈现宽吸收峰。1（* +）衰变的特定途径是质子消除。其发生速率常数k =（2.2±0.2）×10⁴ s⁻¹，生成环状C中心自由基1（-H）（）。后者自由基通过开环（β - 断裂）衰变，估计速率常数约为10⁵ s⁻¹。一个独特的即时产物（以相同速率常数形成）的特征是在λ（max）= 310 nm处有一个窄吸收带，归因于二硫酯官能团的存在。在有氧存在的情况下，310 nm吸收的形成可以被抑制，其原理是C中心环状自由基1（-H）（）与O₂发生反应。310 nm吸收带的消失（速率常数为900 s⁻¹）与二硫酯官能团的水解有关。本研究的另一个方面涉及H（）原子与1的反应，该反应生成一个强吸收的、三电子键合的2σ/1σ自由基阳离子[1（S→S）-H]（+）（λ（max）= 400 nm）。它的形成基于H（）加成到其中一个硫原子上，随后发生β - 断裂、分子内硫 - 硫偶联（构成环收缩），并通过质子化进一步稳定由此形成的S→S键。[1（S→S）-H]（+）以约10⁴ s⁻¹的一级速率常数衰变。通过添加氧气可以抑制其形成，因为氧气在H（）原子与1反应之前将其清除。补充的时间分辨电导率实验提供了有关1（* +）自由基阳离子产率的定量信息、阳离子寿命更长的后续物种、消光系数以及去质子化过程的动力学信息，以及瞬态二硫酯水解后的进一步反应步骤。还根据先前的光化学研究对结果进行了讨论。
