Lundberg Marcus, Siegbahn Per E M
Department of Physics, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden.
J Phys Chem B. 2005 May 26;109(20):10513-20. doi: 10.1021/jp051116q.
Electron-transfer reactions in eight mixed-valence manganese dimers are studied using B3LYP. One of the dimers is a model of the active site of manganese catalase, while another represents a basic building block of the oxygen-evolving complex in photosystem II. The adiabatic reactions are characterized by fully optimized transition states where the single imaginary frequency represents the electron-transfer coordinate. When there is antiferromagnetic coupling between different high-spin centers, electron transfer must be accompanied by a spin transition. Spin transitions are characterized by minimum-energy crossing points between spin surfaces. Three reaction mechanisms have been investigated. First, a single-step reaction where spin flip is concerted with electron transfer. Second, an initial transition to a center with intermediate spin that can be followed by electron transfer. Third, an initial transition to a ferromagnetic state from which the electron can be transferred adiabatically. The complexes prefer the third route with rate-determining barriers ranging from 5.7 kcal/mol to 17.2 kcal/mol for different complexes. The origins of these differences are discussed in terms of oxidation states and ligand environments. Many DFT functionals overestimate charge-transfer interactions, but for the present complexes, the error should be limited because of short Mn-Mn distances.
使用B3LYP方法研究了八个混合价态锰二聚体中的电子转移反应。其中一个二聚体是锰过氧化氢酶活性位点的模型，而另一个代表光系统II中析氧复合物的基本结构单元。绝热反应的特征是具有完全优化的过渡态，其中单一虚频代表电子转移坐标。当不同的高自旋中心之间存在反铁磁耦合时，电子转移必须伴随着自旋转变。自旋转变的特征是自旋表面之间的最低能量交叉点。研究了三种反应机制。第一，自旋翻转与电子转移协同进行的单步反应。第二，初始转变为具有中间自旋的中心，随后可进行电子转移。第三，初始转变为铁磁态，电子可从该铁磁态绝热转移。对于不同的复合物，这些复合物倾向于第三种途径，速率决定势垒范围为5.7千卡/摩尔至17.2千卡/摩尔。根据氧化态和配体环境讨论了这些差异的起源。许多密度泛函理论（DFT）泛函高估了电荷转移相互作用，但对于目前的复合物，由于锰-锰距离较短，误差应该是有限的。
