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首页> 外文期刊>Chemistry: A European journal >Photohydroxylation of 1,4-Benzoquinone in Aqueous Solution Revisited
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Photohydroxylation of 1,4-Benzoquinone in Aqueous Solution Revisited

机译：水溶液中1,4-苯醌的光羟基化

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摘要

In water,photolysis of 1,4-benzoquinone,Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH_2 which are formed with a quantum yield of PHI=0.42,independent of pH and Q concentration.By contrast,the rate of decay of the triplet (gamma_(max)=282 and approx410 nm) which is the precursor of these products increases nonlinearly (k=(2->3.8)x10~6 s~(-1)) with increasing Q concentration ((0.2->10) mM).The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form.These observations are explained by a rapid equilibrium of a monomeric triplet Q~* and an exciplex Q_2~* (K=5500+--1000 M~(-1)).While Q~* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)_3,Q_2~* decays by electron transfer and water addition yielding benzosemiquinone ~·QH and ~·OH adduct radical ~·QOH.The latter enolizes to the 2-hydroxy-1,4-semiquinone radical ~·Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q and HOQ with the concomitant for-mation of ~·QH.On the post-millisecond time scale,that is,when ~·QH has decayed,Ph(OH)_3 is oxidized by Q yielding HOQ and QH_2 as followed by laser flash photolysis with diode array detection.The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow.This shows that there are two pathways to photohydroxylation;a free-radical pathway at high and a non-radical one at low Q concentration.In agreement with this,the yield of Ph(OH)_3 is most pronounced at low Q concentration.In the presence of phosphate buffer,Q~* reacts with H_2PO_4~- giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone OP.The current view that ~·OH is an intermediate in the photohydroxylation of Q has been overturned.This view had been based on the observation of the ~·OH adduct of DMPO when Q is photolyzed in the presence of this spin trap.It is now shown that Q~*/Q_2~* oxidizes DMPO (kapprox=1x10~8 M~(-1) s~(-1)) to its radical cation which subsequently reacts with water.Q~*/Q_2~* react with alcohols by H abstraction (rates in units of M~(-1) s~(-1);methanol (4.2X10~7),ethanol (6.7X10~7),2-propanol (13X10~7) and tertiary butyl alcohol (approx0.2X10~7).DMSO (2.7X10~9) and O_2 (approx2X10~9) act as physical quenchers.
机译：在水中，1,4-苯醌Q的光解会产生等量的2-羟基-1,4-苯醌HOQ和对苯二酚QH_2，其量子产率为PHI = 0.42，与pH和Q浓度无关。相比之下，作为这些产物前体的三重态（gamma_（max）= 282和大约410 nm）的衰减速率随着（k =（2-> 3.8）x10〜6 s〜（-1））的增加而非线性增加。 Q浓度增加（（0.2-> 10）mM）。三重态衰变后通过激光闪光光解法检测到的自由基产量也随着Q浓度增加而增加，但遵循不同的功能形式。这些观察结果通过快速平衡来解释。单体三重态Q〜*和激基复合物Q_2〜*（K = 5500 +-1000 M〜（-1））。Q〜*加水并随后烯化为1,2,4-三羟基苯Ph（OH） _3，Q_2〜*通过电子转移和加水而分解，生成苯并半醌〜·QH和〜·OH加合物自由基〜·QOH。后者烯醇化为2-羟基-1,4-半醌醌〜·Q（OH）H在三重态衰减的时间尺度内，随后被Q和HOQ迅速氧化（微秒时间尺度），同时形成〜·QH。在毫秒后的时间尺度上，即〜·QH衰减时，Ph（OH）_3被Q氧化，生成HOQ和QH_2，然后进行激光闪速光解和二极管阵列检测。pH和Q浓度依赖性反应的速率由停止流独立确定。在低Q浓度下，有两个光羟基化途径;在高Q浓度下有一个自由基途径，而在低Q浓度下有一个非自由基途径。与此相一致，在低Q浓度下，Ph（OH）_3的产量最为明显。 ，Q〜*与H_2PO_4〜-反应生成加合物，随后被Q氧化为2-磷酸-1,4-苯醌OP。目前认为〜·OH是Q光羟基化的中间体该观点是基于对Q为Q时DMPO ~~ OH加合物的观察。现在证明Q〜* / Q_2〜*将DMPO（kapprox = 1x10〜8 M〜（-1）s〜（-1））氧化成其自由基阳离子，随后与之反应Q〜* / Q_2〜*通过H提取与醇反应（速率为M〜（-1）s〜（-1）;甲醇（4.2X10〜7），乙醇（6.7X10〜7），2 -丙醇（13X10〜7）和叔丁醇（约0.2X10〜7）.DMSO（2.7X10〜9）和O_2（约2X10〜9）用作物理淬灭剂。

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