| Research Abstract |
The selective hydration of ions in organic solvents is a key concept for understanding ion-transfer processes across the oil/water interface. Also, it has a fundamental significance for understanding a role of water in hydrophobic cores in proteins. In this study we employed proton NMR and solvent extraction combined with the Karl Fischer method to study the selective hydration of various ions in nitrobenzene (NB).
In proton NMR, the chemical shifts and spin-lattice relaxation times of water protons were measured for the hydrated ions (ClィイD1-ィエD1, BrィイD1-ィエD1, IィイD1-ィエD1, ClOィイD24ィエD2ィイD1-ィエD1, NOィイD23ィエD2ィイD1-ィエD1, SCNィイD1-ィエD1) in deuterated NB. The chemical shifts showed that the selective hydration could be elucidated in terms of the successive reaction mechanism, whereas the rotational correlation times estimated from the spin-lattice relation times were found to be in the order of picoseconds.
In solvent extraction, the hydration numbers (nィイD2hィエD2's) of alkylammonium ions in NB, being little affected y the alkyl chain length, were found to decrease with heightening the class of the alkylammonium ions : nィイD2hィエD2=1.64, 1.04, and 0.66, respectively, for primary, secondary, and tertiary ammonium ions. We have extended the measurements to carboxylic acids and have found that their nィイD2hィエD2-values (ca. 2.6) in NB are hardly affected by the chemical structures.
Based on the above experimental studies, it was recognized that in the transfers of hydrophilic ions at the oil/water interface, water molecules in the oil phase should play a significant role in determining the ion-transfer energies. Then we could propose a new theory of the ion-transfer energy, which was not founded on conventional electrostatic solvation models.
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