Wander Matthew C F, Kubicki James D, Clark Aurora E, Schoonen Martin A A
Department of Chemistry, Washington State University, P.O. Box 644630, Pullman, Washington 99164-4630, USA.
J Phys Chem A. 2009 Feb 12;113(6):1020-5. doi: 10.1021/jp806842f.
Nelsen's four-point method of separating oxidants and reductants has been tested to evaluate its applicability to proton-coupled electron-transfer reactions. An efficient computational method was developed to determine rate-limiting steps in complex, multistep redox reactions. Geochemical redox reactions are rarely single-step, and by identifying the rate-limiting steps, computational time can be greatly reduced. The reaction of superoxide and ferrous oxide was selected as a test case for its simplicity and its importance in environmental radical generation chemistry (Fenton's reaction). Two approaches, one quantum mechanical and the other semiempirical, were compared. In both approaches, hybrid density functional theory (DFT) was used with the B3LYP/6-31+G(d,p) basis set and a polarized continuum model of the solvent to minimize the structures and determine the energies. In the quantum mechanical case, DFT was used to determine both the Gibbs free energies and the values for the intrinsic component of the reorganization energy of possible combinations of reactants and products. In the latter, experimental DeltaG(f) values were combined with calculated intrinsic reorganization energy values. The computational results matched the relative difference in rate barriers between the reduction of superoxide by ferrous iron above and below pH 4.8. In the acidic pH range, the proton is coupled to the electron transfer, whereas in the neutral case, the proton initiates the electron transfer.
尼尔森分离氧化剂和还原剂的四点法已通过测试，以评估其在质子耦合电子转移反应中的适用性。开发了一种高效的计算方法来确定复杂多步氧化还原反应中的限速步骤。地球化学氧化还原反应很少是单步的，通过识别限速步骤，可以大大减少计算时间。选择超氧化物与氧化亚铁的反应作为测试案例，因其简单性及其在环境自由基生成化学（芬顿反应）中的重要性。比较了两种方法，一种是量子力学方法，另一种是半经验方法。在这两种方法中，均使用混合密度泛函理论（DFT）和B3LYP/6-31+G(d,p)基组以及溶剂的极化连续介质模型来优化结构并确定能量。在量子力学情况下，DFT用于确定吉布斯自由能以及反应物和产物可能组合的重组能内在成分的值。在后者中，将实验ΔG(f)值与计算出的内在重组能值相结合。计算结果与pH 4.8以上和以下亚铁还原超氧化物的速率屏障之间的相对差异相匹配。在酸性pH范围内，质子与电子转移耦合，而在中性情况下，质子引发电子转移。
