Investigation of the cation complexation by macrocyclic ethers using $^{13}C$ NMR spin-lattice dipolar relaxation time measurements

Abstract

The binding constants $(K_a}$ of [18]crown-6 ether with $Na^+$ , $K^+$, and $Ba^{2+}$ thiocyanates were determined by $^{13}C{^1H}$ NMR spin-lattice dipolar relaxation time measurements. The observed relaxation times $(T_{obs})$ for $^{13}C$ nuclei are dependent upon the relaxation times of the complexed $(T_{1a})$ and free crown ether $(T_{1f})$, and were measured in [D4]methanol using inversion--recovery measurements in the extreme narrowing limit (75 MHz). The observed $^{13}C$ 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 $1/T_{obs} = Pa/T_{1a} + P_f/T_{1f}$ 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 $Ba^{2+} > K^+ > Na^+$. The $T_1$ measurements within the temperature range of 280-301 K yielded energy barriers for the internal interconversion of the $-O-CH_2-CH_2-O-$ 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.