Abstract:

The binding constants (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 (Tobs) for 13C nuclei are dependent upon the relaxation times of the complexed (T1a) and free crown ether (T1f), and were measured in [D4]methanol using inversion--recovery measurements in the extreme narrowing limit (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.

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