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ID 40790
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creator
Kimura, Tetsunari
Guo, Hao
Furutani, Yuji
NDC
Chemistry
abstract
We demonstrate a powerful spectroscopic technique, surface-enhanced infrared absorption (SEIRA) spectroscopy, not only for detecting host-guest complexes in solution but for examining the relationship between the guest selectivity, complex structure, and solvent effect. We synthesize thiol derivatives of 15-crown-5 and 18-crown-6 [2-(6-mercaptohexyloxy)methyl-15-crown-5 (15C5-C1OC6-SH) and 2-(6-mercaptohexyloxy)methyl-18-crown-6 (18C6-C1OC6-SH)] and adsorb them on gold surfaces through S–Au bonds. The IR difference spectra of the M+•15C5-C1OC6 (M = Li, Na, K, Rb, and Cs) complexes on gold are observed using aqueous solutions of MCl by SEIRA spectroscopy. The spectra show noticeable change in the C–O stretching vibration around 1100 cm–1. The spectral patterns of M+•15C5-C1OC6 are similar for Li+ and Na+, and for K+, Rb+, and Cs+; the interaction between the metal ions and 15C5-C1OC6 changes drastically between Na+ and K+ in the series of alkali metal ions. On the other hand, the equilibrium constant of the complex formation determined by the IR intensity shows clear preference for Na+ ion. We also observe the IR difference spectra of M+•18C6-C1OC6 in methanol and compare them with those in water. The spectral patterns in methanol are almost the same as those in water, but the equilibrium constant in methanol does not show preference for any ion, different from the K+ preference in water. From these findings we attribute the origin of the ion selectivity of 15C5 and 18C6 in solution to the interaction between the metal ions and the crown ethers in the complexes or the solvation energy of free ions. In the case of 15C5-C1OC6 in water, the preference of Na+ over K+, Rb+, and Cs+ can be attributed to the strength of the interaction or the size matching between the metal ions and 15C5-C1OC6; the Na+ selectivity over Li+ ion is dominated by the solvation energy of free ions. For 18C6-C1OC6 in methanol, the equilibrium constant for the complex formation becomes much bigger in methanol than that in water and loses the selectivity in methanol, because the solvation energy in methanol is fairly smaller than that in water, predominating the contribution from the strength of the interaction between the metal ions and 18C6-C1OC6. The IR spectra measured by SEIRA spectroscopy are quite sensitive to the property of host-guest complexes such as the intermolecular interaction, the structure, and the orientation against the gold surface. However, the evidence of the guest selectivity emerges primarily in the intensity of the spectra, rather than band positions or spectral patterns in the IR spectra.
description
This work was partly supported by grants from JSPS to YF (22247024 and 24650203).
journal title
New Journal of Chemistry
volume
Volume 39
issue
Issue 11
start page
8673
end page
8680
date of issued
2015-11-01
publisher
Royal Society of Chemistry
issn
1144-0546
publisher doi
language
eng
nii type
Journal Article
HU type
Journal Articles
DCMI type
text
format
application/pdf
text version
author
rights
Copyright (c) The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.
relation is version of URL
http://doi.org/10.1039/c5nj01787d
department
Graduate School of Science