Conformation of uncomplexed polyether macrocycles. The structure of bis(2'-carboxy-1',3'-xylyl)-24-crown-6 at 113 K

Synthetic molecular receptors for urea. Macrocyclic ligands with intraannular acidic groups and the complexes with urea

“…This conformation was also found in the crystal structure of 2,6-pyrido-24-crown-8-H2CMHC104 (1:2:1)9 and of bis(2-carboxyl-1,3-xylyl)-24-crown-6. 43 It was predicted to be of relatively low steric energy,25,44 but because the water is too small to fill the cavity completely the conformation with an inwardly oriented methylene group, as shown in Figure 3 weighting factor p 0.06 0.04 0.05 extinction g (XI0"7) 1.9 (7) 1.6 (3) 0.5 (3) > <Ff).…”