Mechanisms of the intermolecular nuclear magnetic relaxation dispersion ofthe (CH3)(4)N+ protons in Gd3+ heavy-water solutions. Interest for the theory of magnetic resonance imaging

The relative motion of a tetramethylammonium (CH3)(4)N+ ion with respect to a Gd3+ octoaqua complex, together with the quantum dynamics of the electro nic spin of this lanthanide, is probed by the nuclear magnetic relaxation d ispersion (NMRD) of the (CH3)(4)N+ proton spins. The measured proton resona nce frequencies range between 10 and 800 MHz, A pronounced maximum is obser ved at around 90 MHz. This behavior is interpreted by assuming that the rel ative diffusion of (CH3)(4)N+ and Gd(D2O)(8)(3+) accounts for their repulsi ve potential of mean force, calculated with the help of the hypernetted cha in approximation for two charged hard spheres in discrete, polar, and polar izable water, and by using a detailed picture of the Gd3+ electronic relaxa tion, based on an independent electronic paramagnetic resonance study. The standard dipolar nuclear relaxation formalism of Solomon-Bloembergen, valid for the above frequencies, leads to overall good agreement with the experi mental data without any adjustable parameters. NMRD experiments using probe solutes of well-known spatial dynamics with respect to a Gd3+ complex, can be combined with the Solomon-Bloembergen theory to provide an indirect est imate of the longitudinal electronic relaxation time of this complex. This knowledge is useful in the theory of magnetic resonance imaging relaxivity.