Migliorati Valentina, Serva Alessandra, Sessa Francesco, Lapi Andrea, D'Angelo Paola
Department of Chemistry , University of Rome "La Sapienza" , P.le Aldo Moro 5 , 00185 Rome , Italy.
Istituto CNR di Metodologie Chimiche-IMC, Sezione Meccanismi di Reazione c/o Department of Chemistry , University of Rome "La Sapienza" , P.le Aldo Moro 5 , 00185 Rome , Italy.
J Phys Chem B. 2018 Mar 15;122(10):2779-2791. doi: 10.1021/acs.jpcb.7b12571. Epub 2018 Mar 6.
A synergic approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to investigate diluted (0.1 M) aqueous solutions of two lanthanide ions (Ln), namely, La and Dy, with triflate, nitrate, and bis(trifluoromethylsulfonyl)imide (TfN) as counterions. The different complexing ability of the three anions has been highlighted by the analysis of the MD simulations: TfN does not form inner-sphere complexes, while a small amount of triflate coordinates both the La and Dy cations in their first solvation shell. On the other hand, the nitrate ion is almost absent in the La first coordination sphere, while forming contact ion pairs with Dy. Both lanthanide ions are found to preferentially interact with the water molecules, and the total number of oxygen atoms coordinating the Ln cations in their first solvation sphere is the same in all of the solutions, regardless of whether they belong to water molecules or to the counterion. The presence of counterions in the cation first or second shell changes neither the first shell distance nor the symmetry of the hydration complex formed in solution. The MD results have been confirmed by comparison with the Ln K-edge XAS experimental data, and the quantitative analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the three salt solutions has provided a definite proof of the accuracy of the force field employed in the simulations and of the MD structural result. The anion-water and water-water hydrogen bond lifetimes have been analyzed highlighting the slow down effect of the triflate, nitrate, and TfN anions on the hydrogen bond dynamics in the Ln first solvation shell, with the effect being stronger in the Dy solutions, due to the higher charge density of the Dy ion as compared to La.
一种将分子动力学（MD）模拟与X射线吸收光谱（XAS）相结合的协同方法，已被用于研究两种镧系离子（Ln），即La和Dy，在稀释（0.1 M）水溶液中的情况，其中三氟甲磺酸根、硝酸根和双（三氟甲基磺酰）亚胺（TfN）作为抗衡离子。通过对MD模拟的分析，突出了这三种阴离子不同的络合能力：TfN不形成内球络合物，而少量的三氟甲磺酸根在La和Dy阳离子的第一溶剂化层中配位。另一方面，硝酸根离子在La的第一配位球中几乎不存在，而与Dy形成接触离子对。发现两种镧系离子都优先与水分子相互作用，并且在所有溶液中，配位Ln阳离子的第一溶剂化层中的氧原子总数相同，无论它们属于水分子还是抗衡离子。阳离子第一或第二壳层中抗衡离子的存在既不改变第一壳层距离，也不改变溶液中形成的水合络合物的对称性。通过与Ln K边XAS实验数据比较，证实了MD结果，并且对三种盐溶液的扩展X射线吸收精细结构（EXAFS）光谱的定量分析，为模拟中使用的力场和MD结构结果的准确性提供了明确的证据。分析了阴离子 - 水和水 - 水氢键寿命，突出了三氟甲磺酸根、硝酸根和TfN阴离子对Ln第一溶剂化层中氢键动力学的减缓作用，由于Dy离子与La相比具有更高的电荷密度，这种作用在Dy溶液中更强。
