Department of Chemistry, New York University, New York, USA.
J Phys Chem B. 2009 Dec 31;113(52):16574-89. doi: 10.1021/jp907853p.
The mono-, di-, and tetrahydrates of trifluoromethanesulfonic acid, which contain characteristic H(3)O(+), H(5)O(2)(+), and H(9)O(4)(+) structures, provide model systems for understanding proton transport in materials with high perfluorosulfonic acid density such as perfluorosulfonic acid membranes commonly employed in hydrogen fuel cells. Ab initio molecular dynamics simulations indicate that protons in these solids are predisposed to transfer to the water most strongly bound to sulfonate groups via a Grotthuss-type mechanism, but quickly return to the most solvated defect structure either due to the lack of a nearby species to stabilize the new defect or a preference for the proton to be maximally hydrated. Path integral molecular dynamics of the mono- and dihydrate reveal significant quantum effects that facilitate proton transfer to the "presolvated" water or SO(3)(-) in the first solvation shell and increase the Zundel character of all the defects. These trends are quantified in free energy profiles for each bonding environment. Hydrogen bonding criteria for HOH-OH(2) and HOH-O(3)S are extracted from the two-dimensional potential of mean force. The quantum radial distribution function, radius of gyration, and root-mean-square displacement position correlation function show that the protonic charge is distributed over two or more water molecules. Metastable structural defects with one excess proton shared between two sulfonate groups and another Zundel or Eigen type cation defect are found for the mono- and dihydrate but not for the tetrahydrate crystal. Results for the tetrahydrate native crystal exhibit minor differences at 210 and 250 K. IR spectra are calculated for all native and stable defect structures. Graph theory techniques are used to characterize the chain lengths and ring sizes in the hydrogen bond network. Low conductivities when limited water is present may be attributable to trapping of protons between SO(3)(-) groups and the increased probability that protons transfer to waters bridging two different sulfonate groups.
三氟甲磺酸的单、二和四水合物包含特征性的 H(3)O(+)、H(5)O(2)(+) 和 H(9)O(4)(+) 结构，为理解具有高全氟磺酸密度的材料中的质子传输提供了模型体系，例如在氢燃料电池中常用的全氟磺酸膜。从头算分子动力学模拟表明，这些固体中的质子倾向于通过 Grotthuss 型机制转移到与磺酸盐基团结合最强的水中，但由于缺乏附近的物种来稳定新缺陷，或者质子优先最大程度地水合，质子很快又回到最溶剂化的缺陷结构。一水合物和二水合物的路径积分分子动力学揭示了显著的量子效应，这些效应促进质子向“预溶剂化”水或第一溶剂化壳层中的 SO(3)(-)转移，并增加所有缺陷的 Zundel 特征。这些趋势在每个键合环境的自由能谱中进行了量化。从二维平均力势能中提取出 HOH-OH(2) 和 HOH-O(3)S 的氢键标准。量子径向分布函数、回转半径和均方根位移位置相关函数表明，质子电荷分布在两个或更多水分子上。一水合物和二水合物中发现了具有两个磺酸盐基团之间共享一个多余质子的亚稳结构缺陷，以及另一个 Zundel 或 Eigen 类型阳离子缺陷，但四水合物晶体中没有发现。在 210 和 250 K 时，对四水合物天然晶体的结果存在较小差异。计算了所有天然和稳定缺陷结构的红外光谱。图论技术用于表征氢键网络中的链长和环大小。在有限的水存在时，低电导率可能归因于质子在 SO(3)(-) 基团之间的捕获以及质子转移到桥接两个不同磺酸盐基团的水中的概率增加。
