C-13 NMR CHEMICAL-SHIFTS - A SINGLE RULE TO DETERMINE THE CONFORMATION OF CALIX[4]ARENES

The conformations of calix[4]arenes can be deduced from the C-13 NMR chemical shift of the methylene groups connecting each pair of aromatic rings. Inspection of 24 cases revealed that when the phenol rings beside each methylene are in a syn orientation (i.e., in cone conformations), the methylene signals appear around delta 31, whereas they appear around delta 37 when both phenol rings are anti oriented (i.e., in 1,3-alternate conformations). Steric effects are believed to be the main cause of such large differences.