STEREOCHEMISTRY OF CROWN ETHERS .4. SOLUTION STRUCTURE AND COMPLEXATIONAL BEHAVIOR OF DIBENZO CROWN ETHERS

The preferred conformations and the complexing behaviour of 16- and 17-membered dibenzo crown ethers- were studied by means of both NMR spectroscopy and molecular mechanical calculations. Conclusions could be drawn about the stoichiometry and stability of the alkali metal ion corand complexes. The spin-lattice relaxation times of the crown ether carbon atoms and the relevant complexes are discussed with respect to the intramolecular flexibility of these. species. The intramolecular reorientations detected are reduced considerably in the alkali metal ion complexes. In the preferred conformers of corands 2, 3 and 6, the molecular mechanical calculations prove that the phenyl rings are positioned in a chair conformation at a twist to each other. The same phenyl rings in the crown ethers 1, 4 and 5 are in a boat conformation. These conformations seem to be important for the optimum complexation of the alkali cations.