Rustad James R, Rosso Kevin M, Felmy Andrew R
Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
J Chem Phys. 2004 Apr 22;120(16):7607-15. doi: 10.1063/1.1687318.
We present a molecular model for ferrous-ferric electron transfer in an aqueous solution that accounts for electronic polarizability and exhibits spontaneous cation hydrolysis. An extended Lagrangian technique is introduced for carrying out calculations of electron-transfer barriers in polarizable systems. The model predicts that the diabatic barrier to electron transfer increases with increasing pH, due to stabilization of the Fe3+ by fluctuations in the number of hydroxide ions in its first coordination sphere, in much the same way as the barrier would increase with increasing dielectric constant in the Marcus theory. We have also calculated the effect of pH on the potential of mean force between two hydrolyzing ions in aqueous solution. As expected, increasing pH reduces the potential of mean force between the ferrous and ferric ions in the model system. The magnitudes of the predicted increase in diabatic transfer barrier and the predicted decrease in the potential of mean force nearly cancel each other at the canonical transfer distance of 0.55 nm. Even though hydrolysis is allowed in our calculations, the distribution of reorganization energies has only one maximum and is Gaussian to an excellent approximation, giving a harmonic free energy surface in the reorganization energy F(DeltaE) with a single minimum. There is thus a surprising amount of overlap in electron-transfer reorganization energies for Fe(2+)-Fe(H2O)6(3+), Fe(2+)-Fe(OH)(H2O)5(2+), and Fe(2+)-Fe(OH)2(H2O)+ couples, indicating that fluctuations in hydrolysis state can be viewed on a continuum with other solvent contributions to the reorganization energy. There appears to be little justification for thinking of the transfer rate as arising from the contributions of different hydrolysis states. Electronic structure calculations indicate that Fe(H2O)6(2+)-Fe(OH)n(H2O)(6-n)(3-n)+ complexes interacting through H3O2- bridges do not have large electronic couplings.
我们提出了一种用于水溶液中亚铁 - 铁离子电子转移的分子模型，该模型考虑了电子极化率并表现出自发的阳离子水解。引入了一种扩展拉格朗日技术来计算可极化系统中的电子转移势垒。该模型预测，由于其第一配位球中氢氧根离子数量的波动使Fe3 +稳定，电子转移的非绝热势垒随pH值增加而增加，这与马库斯理论中势垒随介电常数增加而增加的方式非常相似。我们还计算了pH值对水溶液中两个水解离子之间平均力势的影响。正如预期的那样，pH值的增加降低了模型系统中亚铁离子和铁离子之间的平均力势。在0.55 nm的标准转移距离处，预测的非绝热转移势垒增加量和预测的平均力势降低量几乎相互抵消。尽管我们的计算中允许水解，但重组能的分布只有一个最大值，并且非常近似为高斯分布，在重组能F(ΔE)中给出了具有单个最小值的谐振子自由能表面。因此，Fe(2 +)-Fe(H2O)6(3 +)、Fe(2 +)-Fe(OH)(H2O)5(2 +)和Fe(2 +)-Fe(OH)2(H2O)+对的电子转移重组能有惊人的重叠量，表明水解状态的波动可以与其他溶剂对重组能的贡献连续地看待。认为转移速率源于不同水解状态的贡献似乎没有什么依据。电子结构计算表明，通过H3O2 - 桥相互作用的Fe(H2O)6(2 +)-Fe(OH)n(H2O)(6 - n)(3 - n)+配合物没有大的电子耦合。
