用脉冲辐解技术研究菲醌的单电子氧化(英文)

ONE-ELECTRON OXIDATION OF PHENANTHRENEQUINONE STUDIED BY PULSE RADIOLYSIS

采用脉冲辐解技术,以羟基自由基和硫酸根自由基作为氧化剂,研究了9,10-菲醌(PhQ)在水溶液中的单电子氧化行为,获取了瞬态产物的吸收光谱和有关动力学数据。在中性pH条件下,PhQ 与羟基自由基以(1.5±0.2)×1010 dm3·mol-1·s-1的速率反应,生成一个初级瞬态产物。该瞬态产物的吸收光谱呈现出位于370nm的吸收峰和在较长波长下的一个宽吸收带。进一步实验观察到,随着短波长区初级吸收的衰减,可见光区的吸收同步增加,在400nm处形成最大吸收峰,并产生以500nm为中心的宽吸收带,表明次级瞬态产物的形成。PH9.9条件下得到的结果与中性pH的一致。降低pH至2—4,尽管初始瞬态吸收谱也呈现出位于370nm的最大吸收峰,但伴随着初始吸收的哀减,没有观察到可见区域吸收的增加。在中性pH条件下,硫酸根自由基与PhQ反应的速率常数测定为(4.0±0.6)×109dm3·mol-1·s-1,生成的瞬态吸收谱与氢氧自由基反应得到的次级吸收谱一致,呈现以400nm和500nm为中心的两个吸收带。基于光谱的相似性和动力学分析,以及羟基自由基和硫酸根自由基的特性,本工作推断反应机理如下:PhQ羟基自由基反应首先生成OH加成产物,OH加成产物脱水生成阳离子自由基:阳离子自由基办可通过硫酸根自由基的氧化直接产生。

The one-electron oxidation of phenanthrenequinone (PhQ) in aqueous solutions was investigated by pulse radiolysis using hydroxyl radical and sulphate radical as oxidants. Spectral and kinetic properties of the transients formed due to the reaction of PhQ with the oxidants were obtained. The reaction of hydroxyl radical with PhQ results in an initial species with an absorption spectrum centered at 370nm and a weak broad band in the long wavelength regions at neutral pH. The bimolecular rate constant for the formation of initial species was determined to be (1.5±0.2)×1010dm3·mol-1 -s-1. The absorption in the short wavelength region decays with a concomitant increase in the longer wavelength region resulting in a subsequent species with absorption maximum at 400nm and a broad band centered at 500nm. The absorption in visible region increases with increasing pH. Similar transient absorption was obtained at pH 9.9. At pH 4.0, the initial spectrum also shows absorption maximum at 370nm. However, the subsequent absorption spectrum shows a maximum at a shorter wavelength than those found at pHs 7.1 and 9.9. The absorption increase in visible region was not seen at low pH. The transient spectrum obtained at neutral pH by the reaction of sulphate radical with PhQ are similar to the subsequent spectra for the reaction of hydroxyl radical with PhQ showing two bands centered at 400 nm and 500 nm. The bimolecular rate constant was estimated at (4.0±0.6)×109dm3·mol-1-s-1. On the basis of spectral similarity and kinetic analysis, a possible mechanism for the oxidation of PhQ by both hydroxyl radical and sulphate radical is given.