Hydration of Guanidinium: Second Shell Formation at Small Cluster Size

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• 作者：Richard J. Cooper ; Sven Heiles ; Matthew J. DiTucci ; Evan R. Williams
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：July 31, 2014
• 年：2014
• 卷：118
• 期：30
• 页码：5657-5666
• 全文大小：462K
• ISSN：1520-5215

文摘

The structures of hydrated guanidinium, Gdm⁺(H₂O)_n, where n = 1鈥?, were investigated with infrared photodissociation spectroscopy and with theory. The spectral bands in the free O鈥揌 (3600鈥?800 cm^鈥?) and free N鈥揌 (3500鈥?600 cm^鈥?) regions indicate that, for n between 1 and 3, water molecules bind between the NH₂ groups in the plane of the ion forming one hydrogen bond with each amino group. This hydration structure differs from Gdm⁺ in solution, where molecular dynamics simulations suggest that water molecules form linear H-bonds with the amino groups, likely a result of additional water鈥搘ater interactions in solution that compete with the water鈥揼uanidinium interactions. At n = 4, changes in the free O鈥揌 and bonded O鈥揌 (3000鈥?500 cm^鈥?) regions indicate water鈥搘ater H-bonding and thus the onset of a second hydration shell. An inner shell coordination number of n = 3 is remarkably small for a monovalent cation. For Gdm⁺(H₂O)₅, the additional water molecule forms hydrogen bonds to other water molecules and not to the ion. These results indicate that Gdm⁺ is weakly hydrated, and interactions with water molecules occur in the plane of the ion. This study offers the first experimental assignment of structures for small hydrates of Gdm⁺, which provide insights into the unusual physicochemical properties of this ion.