Ion-dipole interactions of macrocyclic ethers using13C dipole-dipole relaxation time measurements. Part XI: The complexation of 1,4,7,10,13,16-hexaoxa-2,6-dioxocyclooctadecane with Ca2+ in CD3OH

The association constants,K _a, of Ca²⁺ complexes with the nonequivalent binding site macrocycle,1,4,7,10,13,16-hexaoxa-2,6-dioxocyclooctadecane, were determined in CD₃OH solution using¹³C dipole-dipole relaxation time,T ₁ ^DD , studies. The measurements ofT ₁ ^DD of the macrocyclic backbone for different stoichiometries (n:m) of complex formation were conducted under extreme narrowing experimental NMR conditions. The general equilibria given with 1/K _aL ₀] ^n+m–1 = (1-nP) ⁿ (1-mP) ^m /P was used for identical cation and macrocyclic ether-ester concentrations in CD₃OH and the association constants found were comparatively small depending on the cyclic ether-ester segments. The logK _a values varied from 1.09 to 0.231 for 1 :1 and from 2.43 to 3.61 for 1 : 2 and from 2.29 to 4.24 for 2 : 1 ligand to cation complex stoichiometries.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August, 1995.