Nuclear and electronic relaxation in lanthanide solutions: (CH3)4N+/Gd3+ repulsive ion pair in D2O

The longitudinal relaxation rate and self-diffusion coefficient of the tetramethylammonium protons are investigated at 400 MHz in D2O solutions of hydrated Gd3+ paramagnetic impurities, without and with complexing NO3− ions. The results are interpreted using the hypernetted chain approximation of the potential of mean force between the repulsive ions, approximated as charged hard spheres in discrete polar and polarizable water. The standard dipolar relaxation formalism of Solomon is valid for the Gd3+ lanthanide, i.e. its electron relaxation time is much longer than the translational correlation time of the interionic Brownian diffusion. The coordination effect by NO3− is analyzed.