The rotational motion and electrnic relaxaton of the Gd(III)aqua complex in water revisited through a full proton relaxivity study of a probe solute

摘要

Recent advances in the design of fast field cycling (FFC)relaxometers make it now possible to explore the nuclear magnetic relaxation dispersion (NMRD)of semidilute nuclei with short relaxation times.The paramagnetic relaxation rate enhancement of the protons of the tetramethylammonium (CH_3)_4N~+ cation due to the intermolecular magnetic dipolar coupling with the electronic spin S=7/2 of [Gd(D_2O)_8]~(3+)in heavy water has been measured between 10 kHz and 800 MHz by combining FFC and standard relaxation techniques.In order to interpret the complete paramagnetic NMRD profile,particularly in teh low field region,two previously neglected features are taken into account:(i)The evolution beyong the Redfeld limit of the electronic relaxation of the spin S is obtained from accuate Monte Carlo simulations.(ii)The time flucturation of the static zero field spliting (ZFS)is attributed not only to the usual global Brownian rotational diffusion of the complex,but also to the rearrangement of teh water moelcules inteh first hydration shell of the Gd~(3+)ion via 90 deg pseudorotatons [Th.Kowall et al.,J.Phys.Chem.99,13078 (1995)].To calculate the longitudinal electronic relaxation function G_(||)(t)of the Gd~(3+)ion,its static and transient ZFS parameters in the aqua complex as well as the correlation times of the Brownian rotation adn vibrations of this complex are needed.We use the values of these parameters derived froman independent multiple frequency and temperature study fo the full electronic paramagnetic resonance spectra of Gd~(3+)in light water H_2O,for magnetic fields where the Redfield limit applies.The predicted NMRD profile is in excellent global agreement with experiment overthe whole proton frequency range,especially if the correlation times governing the rotational dynamics of the aqua complex are slighlty increased to account forhte higher viscosity of D_2O with respect to H_2O.