Petersen Matt K, Voth Gregory A
Department of Chemistry and Center for Biophysical Modeling and Simulation, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112-0850, USA.
J Phys Chem B. 2006 Sep 21;110(37):18594-600. doi: 10.1021/jp062719k.
The solvation and transport properties of the sulfonate-hydronium ion pair have been studied in hydrated Nafion through molecular dynamics simulation. Explicit proton and charge delocalization of the excess proton transport, via the Grotthuss hopping mechanism, were treated using the self-consistent multistate empirical valence bond (SCI-MS-EVB) method. The nature of the sulfonate-hydronium ion pair was characterized through analysis of free-energy profiles. It was found that, in general, the excess proton is solvated between two water molecules of a Zundel moiety while in the contact ion pair position, but then it transitions to an Eigen-like configuration in the solvent-separated pair position. Furthermore, the positive charge associated with the excess proton passes between the contact and solvent-separated ion pair positions through the Grotthuss mechanism rather than simple vehicular diffusion. The total proton diffusion was decomposed into vehicular and Grotthuss components and were found to be of the same relative magnitude, but with a strong negative correlation resulting in a smaller overall diffusion. Correlated motions between the ion pair were examined through the distinct portion of the van Hove correlation function, and a characteristic time scale of approximately 425 ps was observed. Additionally, the association of the hydrated proton with the hydrophobic polymer backbone suggests its amphiphile-like behavior (see Acc. Chem. Res. 2006, 39, 143; Phys. Rev. 1954, 95, 249; J. Chem. Phys. 2005, 123, 084309).
通过分子动力学模拟研究了磺酸根 - 水合氢离子对在水合Nafion中的溶剂化和传输性质。使用自洽多态经验价键（SCI-MS-EVB）方法处理了通过Grotthuss跳跃机制进行的过量质子传输中的显式质子和电荷离域。通过分析自由能分布来表征磺酸根 - 水合氢离子对的性质。结果发现，一般来说，过量质子在接触离子对位置时被溶剂化在Zundel部分的两个水分子之间，但在溶剂分离对位置时会转变为类似Eigen的构型。此外，与过量质子相关的正电荷通过Grotthuss机制在接触离子对和溶剂分离离子对位置之间传递，而不是简单的载体扩散。总质子扩散被分解为载体扩散和Grotthuss扩散分量，发现它们具有相同的相对大小，但具有很强的负相关性，导致整体扩散较小。通过van Hove关联函数的不同部分研究了离子对之间的相关运动，观察到特征时间尺度约为425 ps。此外，水合质子与疏水聚合物主链的缔合表明其具有两亲性类似行为（见《化学研究述评》2006年，39卷，143页；《物理评论》1954年，95卷，249页；《化学物理杂志》2005年，123卷，084309页）。
