Akademik Veri Yönetim Sistemi
TURKISH JOURNAL OF CHEMISTRY, cilt.30, sa.3, ss.261-268, 2006 (SCI-Expanded)
The binding constants (K-a) of [18] crown-6 ether with Na+, K+, and Ba2+ thiocyanates were determined by C-13{H-1} NMR spin-lattice dipolar relaxation time measurements. The observed relaxation times (T-obs) for C-13 nuclei are dependent upon the relaxation times of the complexed (T-1a) and free crown ether (T-1f), andw ere measured in [D4]methanol using inversion-recovery measurements in the extreme narrowing limit ( 75 MHz). The observed C-13 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/T-obs = P-a/T-1a + P-f/T-1f was used to estimate K-a for the n:m stoichiometry of the cation complexes in [D4] methanol and were found to run in the order Ba2+ > K+ > Na+. The T-1 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.