Bankura Arindam, Chandra Amalendu
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India.
J Chem Phys. 2015 Jan 28;142(4):044701. doi: 10.1063/1.4905495.
The dynamics of proton transfer (PT) through hydrogen bonds in a two-dimensional water layer confined between two graphene sheets at room temperature are investigated through ab initio and quantum-classical simulations. The excess proton is found to be mostly solvated as an Eigen cation where the hydronium ion donates three hydrogen bonds to the neighboring water molecules. In the solvation shell of the hydronium ion, the three coordinated water molecules with two donor hydrogen bonds are found to be properly presolvated to accept a proton. Although no hydrogen bond needs to be broken for transfer of a proton to such presolvated water molecules from the hydronium ion, the PT rate is still found to be not as fast as it is for one-dimensional chains. Here, the PT is slowed down as the probability of finding a water with two donor hydrogen bonds in the solvation shell of the hydronium ion is found to be only 25%-30%. The hydroxide ion is found to be solvated mainly as a complex anion where it accepts four H-bonds through its oxygen atom and the hydrogen atom of the hydroxide ion remains free all the time. Here, the presolvation of the hydroxide ion to accept a proton requires that one of its hydrogen bonds is broken and the proton comes from a neighboring water molecule with two acceptor and one donor hydrogen bonds. The coordination number reduction by breaking of a hydrogen bond is a slow process, and also the population of water molecules with two acceptor and one donor hydrogen bonds is only 20%-25% of the total number of water molecules. All these factors together tend to slow down the hydroxide ion migration rate in two-dimensional water layers compared to that in three-dimensional bulk water.
通过从头算和量子经典模拟，研究了室温下二维水层中质子通过氢键在两个石墨烯片层之间转移的动力学。发现过量质子主要以特征阳离子的形式溶剂化，其中水合氢离子向相邻水分子提供三个氢键。在水合氢离子的溶剂化壳层中，发现具有两个供体氢键的三个配位水分子被适当预溶剂化以接受质子。尽管从水合氢离子向这种预溶剂化水分子转移质子时不需要破坏氢键，但发现质子转移速率仍不如在一维链中那么快。在这里，质子转移减慢是因为在水合氢离子的溶剂化壳层中找到具有两个供体氢键的水的概率仅为25%-30%。发现氢氧根离子主要以复合阴离子的形式溶剂化，其中它通过其氧原子接受四个氢键，氢氧根离子的氢原子始终保持自由。在这里，氢氧根离子预溶剂化以接受质子需要破坏其一个氢键，并且质子来自具有两个受体和一个供体氢键的相邻水分子。通过破坏氢键降低配位数是一个缓慢的过程，并且具有两个受体和一个供体氢键的水分子数量仅占水分子总数的20%-25%。与三维体相水中相比，所有这些因素共同导致二维水层中氢氧根离子的迁移速率减慢。
