The trinuclear [Gd-3(H(-3)taci)(2)(H2O)(6)](3+) complex has been characterized in aqueous solution as a model compound from the point of view of MRI : the parameters that affect proton relaxivity have been determined in a combined variable temperature, pressure, and multiple-field O-17 NMR, EPR,and NMRD study. The solution structure of the complex was found to be the same as in solid state : the total coordination number of the lanthanide(IB) ion is 8 with two inner-sphere water molecules. EPR measurements proved a strong intramolecular dipole-dipole interaction between Gd(III) electron spins. This mechanism dominates electron spin relaxation at high magnetic fields (B > 5 T). its proportion to the overall relaxation decreases with decreasing magnetic field and becomes a minor term at fields used in MRI. Consequently, it cannot increase the electronic relaxation rates to such an extent that they limit proton relaxivity. [Gd-3(H(-3)taci)(2)(H2O)(6)](3+) undergoes a relatively slow water exchange (k(ex)(299) = (1.1 +/- 0.2) x 10(7) s(-1)) compared to the Gd(III) aqua ion, while the mechanism is much more associatively activated as shown by the activation Volume (Delta V-double dagger = (-12.7 +/- 1.5) cm(3) mol(-1)). The lower exchange rate, as compared to [Gd(H2O)(8)](3+) and [Gd(PDTA)(H2O)(2)](-), can be explained with the higher rigidity of the [Gd-3(H(-3)taci)(2)(H2O)(6)](3+) which considerably slows down the transition from the eight-coordinate reactant to the nine-coordinate transition state. The unexpectedly low relational correlation time of the complex is interpreted in terms of a spherical structure with a large hydrophobic surface avoiding the formation of a substantial hydration sphere around [Gd-3(H(-3)taci)(2)(H2O)(6)](3+).