Villa Alessandra, Hess Berk, Saint-Martin Humberto
Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
J Phys Chem B. 2009 May 21;113(20):7270-81. doi: 10.1021/jp8097445.
Aqueous solutions of a light (Nd3+), a middle (Gd3+), and a heavy (Yb3+) lanthanide ion were studied using ab initio based flexible and polarizable analytical potentials in classical molecular dynamics simulations to describe their thermodynamic, structural, and dynamic features. To avoid the spurious demise of O-H bonds, it was necessary to reparametrize an existing water model, which resulted in an improved description of pure water. The good agreement of the results from the simulations with the experimental hydration enthalpies, the Ln(III)-water radial distribution functions, and the water-exchange rates validated the potentials, though the r(Ln-Ow) distances were 6% longer than the experimentally determined values. A nona-coordinated state was found for Nd3+ in 95% of the simulation, with a tricapped trigonal prism (TCTP) geometry; the corresponding water-exchange mechanism was found to be of dissociative interchange (Id) character through a short-lived octa-coordinated transition state in a square antiprism (SQA) geometry. An octa-coordinated state in SQA geometry was found for Yb3+ in 99% of the simulation, and the observed exchange events exhibited characteristics of an interchange (I) mechanism. For Gd3+ an equilibrium was observed between 8-fold SQA and 9-fold TCTP coordinated states that was maintained by the frequent exchange of a water molecule from the first hydration shell with the bulk, thus producing significant deviations from the ideal geometries, and a fast exchange rate. Though strong water-water interactions prevented a full alignment of the dipoles to the ion's electric field, the screening was found large enough as to limit its range to 5 A; water molecules further apart from the ion were found to have the same dipole as the molecules in the bulk, and a random orientation. The interplay among the water-ion and the water-water interactions determined the different coordination numbers and the different dynamics of the water exchange in the first hydration shell for each ion.
在经典分子动力学模拟中，使用基于从头算的灵活且可极化的分析势，研究了轻镧系离子（Nd3+）、中镧系离子（Gd3+）和重镧系离子（Yb3+）的水溶液，以描述它们的热力学、结构和动力学特征。为避免O-H键的虚假断裂，有必要重新参数化现有的水模型，这使得对纯水的描述得到了改进。模拟结果与实验水化焓、Ln(III)-水径向分布函数以及水交换速率之间的良好一致性验证了这些势，尽管r(Ln-Ow)距离比实验测定值长6%。在95%的模拟中发现Nd3+处于九配位状态，具有三帽三角棱柱（TCTP）几何结构；相应地，发现水交换机制具有解离交换（Id）特征，通过方形反棱柱（SQA）几何结构中的短寿命八配位过渡态进行。在99%的模拟中发现Yb3+处于SQA几何结构的八配位状态，观察到的交换事件表现出交换（I）机制的特征。对于Gd3+，观察到八重SQA和九重TCTP配位状态之间的平衡，这种平衡通过第一个水化层中的水分子与本体频繁交换来维持，从而产生与理想几何结构的显著偏差以及快速的交换速率。尽管强的水-水相互作用阻止了偶极子完全与离子电场对齐，但发现屏蔽作用足够大，足以将其范围限制在5 Å；发现离离子更远的水分子与本体中的分子具有相同的偶极子，且取向随机。水-离子相互作用和水-水相互作用之间的相互作用决定了每个离子在第一个水化层中不同的配位数和不同的水交换动力学。
