An 18-membered macrocyclic DTPA-bis(amide) ligand (DTPA = diethylenetriaminepentaacetic acid) containing a heteroatom in the amide link has been prepared via the condensation of DTPA-dianhydride and diethylenetriamine. The solution structures of the two isomeric pairs present in the Ln(III) complexes of DTPA-dien have been investigated by H-1 NMR. One enantiomeric pair exhibits slow exchange on the NMR time scale at low temperatures (0-25 degrees C) and "dynamic" behavior at higher temperatures. The other isomeric pair exhibits an unusual "static" behavior; exchange remains slow even at 95 degrees C. Peak assignments for the Eu(DTPA-dien) spectra are given based on deuteration studies, 2D COSY spectroscopy, and 2D EXSY spectroscopy. 2D EXSY spectroscopy at several temperatures and mixing times showed that Delta G(double dagger) (299) for the "dynamic" isomerization is 57.5 +/- 0.3 kJ/mol, and that the "dynamic" isomer is an intermediate for the "static" isomerization, which occurs with a change in backbone amine chirality. The structures of the lanthanum(III) and europium(III) DTPA-dien complexes have been determined by X-ray analysis. [La(DTPA-dienH(+))H2O](2)(CF3SO3-)(2).18H(2)O (I) crystallizes as a carboxylate-bridged dimer about a center of inversion in the orthorhombic space group Pbca with a = 12.626(2) Angstrom, b = 21.405(3) Angstrom, c = 26.422(9) Angstrom, and Z = 8. Each lanthanum ion is 11-coordinate with octadentate ligand coordination, an eta(2) bridging carboxylate, and one water. [Eu(DTPA-dienH(+))](4)(CF3SO3-)(4).6NaCF(3)SO(3).2OH(2)O (II) crystallizes as a carboxylate-bridged tetramer with two crystallographically independent Eu(III) positions (Z = 8 for each) in the monoclinic space group C2/c : a = 30.94(1) Angstrom, b = 23.456(3) Angstrom, c = 22.611(4) Angstrom, beta = 105.78(2)degrees. The coordination geometries about Eu1 and Eu2 are nearly identical and are described as a nine-coordinate tricapped trigonal prism with octadentate ligand coordination plus an eta(1) bridging carboxylate. The tendency to oligomerize is attributed to the constraints imposed by the macrocycle and the hydrogen bonding available with the link heteroatom The structrual differences between the two complexes are attributed to a difference in La(III) and Eu(III) ionic size. The solution structure of the "dynamic" isomer is found to be the same as the monomer unit of the crystal structures, and the "static" isomer is similar, save for a change in one terminal backbone nitrogens’ chirality.