Theoretical Modeling of Magnesium Ion Imprints in the Raman Scattering of Water

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• 作者：Josef Kapitn ; Martin Dransk ; Jakub Kaminsk ; Ladislav Benda ; Petr Bou
• 刊名：Journal of Physical Chemistry B
• 出版年：2010
• 出版时间：March 18, 2010
• 年：2010
• 卷：114
• 期：10
• 页码：3574-3582
• 全文大小：261K
• 年卷期：v.114,no.10(March 18, 2010)
• ISSN：1520-5207

文摘

Hydration envelopes of metallic ions significantly influence their chemical properties and biological functioning. Previous computational studies, nuclear magnetic resonance (NMR), and vibrational spectra indicated a strong affinity of the Mg²⁺ cation to water. We find it interesting that, although monatomic ions do not vibrate themselves, they cause notable changes in the water Raman signal. Therefore, in this study, we used a combination of Raman spectroscopy and computer modeling to analyze the magnesium hydration shell and origin of the signal. In the measured spectra of several salts (LiCl, NaCl, KCl, MgCl₂, CaCl₂, MgBr₂, and MgI₂ water solutions), only the spectroscopic imprint of the hydrated Mg²⁺ cation could clearly be identified as an exceptionally distinct peak at 355 cm⁻¹. The assignment of this band to the Mg−O stretching motion could be confirmed on the basis of several models involving quantum chemical computations on metal/water clusters. Minor Raman spectral features could also be explained. Ab initio and Fourier transform (FT) techniques coupled with the Car−Parrinello molecular dynamics were adapted to provide the spectra from dynamical trajectories. The results suggest that even in concentrated solutions magnesium preferentially forms a [Mg(H₂O)₆]²⁺ complex of a nearly octahedral symmetry; nevertheless, the Raman signal is primarily associated with the relatively strong metal−H₂O bond. Partially covalent character of the Mg−O bond was confirmed by a natural bond orbital analysis. Computations on hydrated chlorine anion did not provide a specific signal. The FT techniques gave good spectral profiles in the high-frequency region, whereas the lowest-wavenumber vibrations were better reproduced by the cluster models. Both dynamical and cluster computational models provided a useful link between spectral shapes and specific ion−water interactions.