AZA CROWN ETHERS - C-13 AND H-1-NMR STUDIES OF RING CONFORMATIONS AND STEREOCHEMICALLY DEPENDENT SHIFTS IN N-SUBSTITUTED DIBENZODIAZA-18-CROWN-6 ETHERS AND A 9-MEMBERED ANALOG

Remarkably large (> 10 ppm) C-13 NMR chemical shift differences are observed in solution and in the solid phase between ortho aromatic carbons of structurally analogous dibenzodiaza-18-crown-6 ethers and their 9-membered ring counterparts. On the basis of geometries deduced from H-1H-1 NOESY results, the upfield C-13 chemical shifts in the 18-membered systems are attributed to cis coplanar interactions between the terminal carbons of these torsional networks. Overall mobilities of 9- and 18-membered ring systems are examined via C-13 spin-lattice relaxation times.