Bylaska Eric J, Valiev Marat, Rustad James R, Weare John H
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA.
J Chem Phys. 2007 Mar 14;126(10):104505. doi: 10.1063/1.2566868.
First principles simulations of the hydration shells surrounding Al3+ ions are reported for temperatures near 300 degrees C. The predicted six water molecules in the octahedral first hydration shell were found to be trigonally coordinated via hydrogen bonds to 12 s shell water molecules in agreement with the putative structure used to analyze the x-ray data, but in disagreement with the results reported from conventional molecular dynamics using two-and three-body potentials. Bond lengths and angles of the water molecules in the first and second hydration shells and the average radii of these shells also agreed very well with the results of the x-ray analysis. Water transfers into and out of the second solvation shell were observed to occur on a picosecond time scale via a dissociative mechanism. Beyond the second shell the bonding pattern substantially returned to the tetrahedral structure of bulk water. Most of the simulations were done with 64 solvating water molecules (20 ps). Limited simulations with 128 water molecules (7 ps) were also carried out. Results agreed as to the general structure of the solvation region and were essentially the same for the first and second shell. However, there were differences in hydrogen bonding and Al-O radial distribution function in the region just beyond the second shell. At the end of the second shell a nearly zero minimum in the Al-O radial distribution was found for the 128 water system. This minimum is less pronounced minimum found for the 64 water system, which may indicate that sizes larger than 64 may be required to reliably predict behavior in this region.
报道了在接近300摄氏度的温度下，围绕Al3+离子的水合壳层的第一性原理模拟。发现在八面体第一水合壳层中预测的六个水分子通过氢键与12个s壳层水分子呈三角配位，这与用于分析x射线数据的假定结构一致，但与使用两体和三体势的传统分子动力学报告的结果不一致。第一和第二水合壳层中水分子的键长和键角以及这些壳层的平均半径也与x射线分析结果非常吻合。观察到水通过解离机制在皮秒时间尺度上进出第二溶剂化壳层。在第二壳层之外，键合模式基本上恢复到 bulk 水的四面体结构。大多数模拟是用64个溶剂化水分子进行的（20皮秒）。也进行了用128个水分子的有限模拟（7皮秒）。关于溶剂化区域的一般结构，结果是一致的，并且第一和第二壳层的结果基本相同。然而，在第二壳层之外的区域，氢键和Al-O径向分布函数存在差异。在第二壳层末端，对于128个水的体系，Al-O径向分布中发现了一个几乎为零的最小值。这个最小值在64个水的体系中不太明显，这可能表明可能需要大于64的尺寸才能可靠地预测该区域的行为。
