Molecular motion in crown ethers. Application of C-13 and H-2 NMR to the study of 4-carboxybenzo-24-crown-8 ether and its KNCS complex in solution and in the solid phase

Large-amplitude solid phase molecular motion has been detected in the macrocyclic ring of the title crown either via C-13 CPMAS NMR. To study the details of the dynamic processes, two selectively deuterated d(4) derivatives have been prepared and examined via H-2 NMR as a function of temperature. A phase change occurring around 277 K has been verified by differential scanning calorimetry (DSC) and a model for the motional processes has been developed involving equivalent two-site flips of the CD2 groups. The amplitude of the CD2 motions apparently decreases the closer the group is to the aromatic ring. The influence of KNCS complexation on the C-13 CPMAS spectrum and on C-13 spin lattice relaxation times in solution has been explored.