Evidence for large inner reorganization energies in the reduction of diaryl disulfides: Toward a mechanistic link between concerted and stepwise dissociative electron transfers?
Evidence for large inner reorganization energies in the reduction of diaryl disulfides: Toward a mechanistic link between concerted and stepwise dissociative electron transfers?
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DOI：
10.1021/ja983374p
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发表时间：
1999-03-03
影响因子：
15
通讯作者：
Maran, F
中科院分区：
文献类型：
--
作者：
Daasbjerg, K;Jensen, H;Maran, F
键断裂通常伴随着电子被带有合适离去基团的底物吸收。1通常，键断裂发生在电子转移（ET）到受体分子（AB）之后（等式1）。但当离去基团对氧化相对稳定和/或键较弱时，例如与烷基卤1，2或过氧化物，3 ET和键断裂可以协同进行，并且自由基A·和阴离子B-在一个步骤中形成（等式2）。2除了ET步骤的化学差异外，两种ET机制不同的一个重要方面是重组能。逐步机制（方程式1）通常表示为自由基阴离子AB·-的形成没有相对于其前体的显著结构重组，因此将一个电子注入AB的动力学被视为主要由溶剂重组控制。对于纯解离的ET（等式2），情况并非如此，其中AB键的键解离能（BDE）的1/4贡献于内部重组能。2因此，表示为溶剂和内部贡献之和（Δ G 0 q）Δ G 0，s q+ Δ G 0，i q）的本征势垒可以相当大，对活化自由能ΔGq具有显著影响。[4]已经表明，两种竞争机制之间的转变可以通过改变电子供体（电极或溶液物质）的还原性质来诱导。图3c，5然而，当一个逐步的ET过程涉及到一个重要的内部重组时，会发生什么并不那么清楚。这个问题并不是微不足道的。事实上，内部重组的贡献越来越大，可以看作是上述两种机制之间的联系，
Bond fragmentation often accompanies electron uptake by substrates bearing suitable leaving groups. 1 Usually, bond fragmentation follows the electron transfer (ET) to the acceptor molecule (AB)(eq 1). But when the leaving group is relatively stable to oxidation and/or the bond is weak, such as with alkyl halides1, 2 or peroxides, 3 ET and bond cleavage can be concerted, and the radical A• and the anion B-form in a single step (eq 2). 2Besides the chemical difference of the ET step, an important aspect by which the two ET mechanisms differ concerns the reorganization energies. The stepwise mechanism (eq 1) is usually represented as one in which the radical anion AB•-forms without significant structural reorganization with respect to its precursor so that the kinetics of the injection of one electron into AB is viewed as being ruled primarily by solvent reorganization. This is not the case for a purely dissociative ET (eq 2), where 1/4 of the bond dissociation energy (BDE) of the AB bond contributes to the inner reorganization energy. 2 As a consequence, the intrinsic barrier, expressed as the sum of solvent and inner contributions (ΔG0 q) ΔG0, s q+ ΔG0, i q), can be quite large having a significant effect on the activation free energy ΔGq. 4 It has already been shown that the transition between the two competitive mechanisms can be induced by changing the reducing properties of the electron donor (an electrode or a solution species). 3c, 5 However, it is not so clear what happens when a stepwise ET process involves a significant inner reorganization. The problem is not trivial. In fact, an increasing contribution from inner reorganization may be viewed as a link between the above two mechanisms and thus to