Probing the Transition from Hydrophilic to Hydrophobic Solvation with Atomic Scale Resolution

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• 作者：Van-Thai Pham ; Thomas J. Penfold ; Renske M. van der Veen ; Frederico Lima ; Amal El Nahhas ; Steve L. Johnson ; Paul Beaud ; Rafael Abela ; Christian Bressler ; Ivano Tavernelli ; Christopher J. Milne ; Majed Chergui
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：August 17, 2011
• 年：2011
• 卷：133
• 期：32
• 页码：12740-12748
• 全文大小：966K
• 年卷期：v.133,no.32(August 17, 2011)
• ISSN：1520-5126

文摘

Picosecond and femtosecond X-ray absorption spectroscopy is used to probe the changes of the solvent shell structure upon electron abstraction of aqueous iodide using an ultrashort laser pulse. The transient L_1,3 edge EXAFS at 50 ps time delay points to the formation of an expanded water cavity around the iodine atom, in good agreement with classical and quantum mechanical/molecular mechanics (QM/MM) molecular dynamics (MD) simulations. These also show that while the hydrogen atoms pointed toward iodide, they predominantly point toward the bulk solvent in the case of iodine, suggesting a hydrophobic behavior. This is further confirmed by quantum chemical (QC) calculations of I^{鈥?/sup>/I0(H₂O)_n=1鈥? clusters. The L₁ edge sub-picosecond spectra point to the existence of a transient species that is not present at 50 ps. The QC calculations and the QM/MM MD simulations identify this transient species as an I0(OH₂) complex inside the cavity. The simulations show that upon electron abstraction most of the water molecules move away from iodine, while one comes closer to form the complex that lives for 3鈥? ps. This time is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform an H-bond network. Only then is the interaction with the solvation shell strong enough to pull the water molecule of the complex toward the bulk solvent. Overall, much of the behavior at early times is determined by the reorientational dynamics of water molecules and the formation of a complete network of hydrogen bonded molecules in the first solvation shell.}