Investigation of the Cation Complexation by Macrocyclic Ethers using 13C NMR Spin--Lattice Dipolar Relaxation Time Measurements

Authors : Çakıl ERK, Matthias HEYDENREICH, And Erich KLEINPETER

Pages : 261-268

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Publication Date : 0000-00-00

Article Type : Research Paper

Abstract :The binding constants insert ignore into journalissuearticles values(Ka); of [18]crown-6 ether with Na+ , K+, and Ba2+ thiocyanates were determined by 13C{1H} NMR spin--lattice dipolar relaxation time measurements. The observed relaxation times insert ignore into journalissuearticles values(Tobs); for 13C nuclei are dependent upon the relaxation times of the complexed insert ignore into journalissuearticles values(T1a); and free crown ether insert ignore into journalissuearticles values(T1f);, and were measured in [D4]methanol using inversion--recovery measurements in the extreme narrowing limit insert ignore into journalissuearticles values(75 MHz);. The observed 13C relaxation times of the metal complexes were found to be smaller than those of the cation-free macrocyclic ether due to reduced internal flexibility of the macrocycles in the complexes. The relationship 1/Tobs = Pa/T1a + Pf/T1f was used to estimate Ka for the n:m stoichiometry of the cation complexes in [D4]methanol and were found to run in the order Ba2+ > K+ > Na+. The T1 measurements within the temperature range of 280--301 K yielded energy barriers for the internal interconversion of the --O--CH2--CH2--O-- structural fragments in free and complexed [18]crown-6 ether. The results indicated that the energy barriers of complexed crown ether are lower than those of the cation-free molecule, indicating the stabilization of preferred conformations in the cation-complexed crown ethers.
Keywords : Turk J Chem, 30, 2006, 261 268 Turk J Chem, vol 30, iss 3