We present the results of new and previously published 17O NMR, EPR, and NMRD studies of aqueous solutions of the Gd3+ octaaqua ion and the commercial MRI contrast agents [Gd(DTPA)(H2O)]2- (MAGNEVIST, Schering AG, DTPA = 1,1,4,7,7-pentakis(carboxymethyl)-1,4,7-triazaheptane), [Gd(DTPA-BMA)(H2O)] (OMNISCAN, Sanofi Nycomed, DTPA-BMA = 1,7-bis[(N-methylcarbamoyl)methyl]-1,4,7-tris(carboxymethyl)-1,4,7-triazaheptane), and [Gd(DOTA)(H2O)]- (DOTAREM, Guerbet, DOTA = 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane). High-field EPR measurements at different concentrations give evidence of an intermolecular dipole−dipole electronic relaxation mechanism that has not previously been described for Gd3+ complexes. For the first time, the experimental data from the three techniques for each complex have been treated using a self-consistent theoretical model in a simultaneous multiple parameter least-squares fitting procedure. The lower quality of the fits compared to separate fits of the data for each of the three techniques shows that the increase in the number of adjustable parameters is outweighed by the increased constraint on the fits. The parameters governing the relaxivity of the complexes are thus determined with greater confidence than previously possible. The same approach was used to study two dimeric Gd3+ complexes [pip{Gd(DO3A)(H2O)}2] and [bisoxa{Gd(DO3A)(H2O)}2] (pip(DO3A)2 = bis(1,4-(1-(carboxymethyl)-1,4,7,10-tetraaza-4,7,10-tris(carboxymethyl)-1-cyclododecyl-1,4-diazacyclohexane, bisoxa(DO3A)2 = bis(1,4-(1-(carboxymethyl)-1,4,7,10-tetraaza-4,7,10-tris(carboxymethyl)-1-cyclododecyl))-1,10-diaza-3,6-dioxadecane) that are being developed as potential second-generation MRI contrast agents. These dimeric complexes are expected to have higher relaxivities than the monomeric contrast agents, due to their longer rotational correlation times. The results of this study show that further relaxivity gain for these complexes will be hindered by the slow rate of water exchange on the complexes. High-field EPR measurements suggest that there is a previously unrecorded intramolecular dipole−dipole mechanism of electronic relaxation, but that this additional contribution to electronic relaxation is of minor importance compared to the limiting effect of water exchange rates in the determination of proton relaxivity in MRI applications.

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A variable-temperature, -pressure, and -ionic strength 1 H NMR study of the DOTA complexes of different trivalent cations (Sc, Y, La, Ce f Lu) (DOTA ) 1,4,7,10-tetraaza-1,4,7,10-tetrakis(carboxymethyl)cyclododecane) yielded data that are in contradiction with the hitherto used model of only two enantiomeric pairs of diastereoisomers that differ in the ligand conformations. A two-isomer equilibrium cannot explain the newly observed apparent reversal of the isomer ratio at the end of the series. As both conformers may lose their inner sphere water molecule, a coordination equilibrium may be superimposed on this conformational equilibrium, as shown by large positive reaction volumes for the isomerization of [Ln(DOTA)(

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We determined the structure of the hydrated Cu(II) complex by both neutron diffraction and first-principles molecular dynamics. In contrast with the generally accepted picture, which assumes an octahedrally solvated Cu(II) ion, our experimental and theoretical results favor fivefold coordination. The simulation reveals that the solvated complex undergoes frequent transformations between square pyramidal and trigonal bipyramidal configurations. We argue that this picture is also consistent with experimental data obtained previously by visible near-infrared absorption, x-ray absorption near-edge structure, and nuclear magnetic resonance methods. The preference of the Cu(II) ion for fivefold instead of sixfold coordination, which occurs for other cations of comparable charge and size, results from a Jahn-Teller destabilization of the octahedral complex.

We report the nanoscale loading and confinement of aquated Gd3+n-ion clusters within ultra-short single-walled carbon nanotubes (US-tubes); these Gd3+n@US-tube species are linear superparamagnetic molecular magnets with Magnetic Resonance Imaging (MRI) efficacies 40 to 90 times larger than any Gd3+-based contrast agent (CA) in current clinical use.

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