Molecular Motion and Relaxation in Free-Radical Solutions of Benzene, Toluene, and Some Ethers as Studied by Dynamic Nuclear Polarization

Abstract

Magnetic double-resonance methods have been used to investigate the spectrum of motion, the proton and electron spin relaxation in organic solutions of free radicals. Studies of both frequency and temperature dependences of the dynamic nuclear polarization and the relaxation times yield information on the solvent-nucleus—free-radical interactions and on the molecular motion. Measurements in very weak magnetic fields (15 G) realize the conditions of ``extreme narrowing,'' data at 1070 and 3230 G fall into the main region of decay of the Overhauser effect. The results in radical solutions of benzene, toluene, dimethoxymethane, dimethoxyethane, and diethoxymethane can be explained by a pure dipole—dipole interaction governed by translational random motions of the spin-carrying molecules. Correlation times, activation energies, and other parameters are evaluated. An influence of different free radicals (molecules and ions) could obviously not be observed. Toluene at low temperatures displays an exceptional behavior. Some consequences for the phenomena of Overhauser polarization, proton, and electron spin—lattice relaxation in general are discussed. This especially includes the high-frequency limit of the dynamic nuclear polarization and the dependences of both leakage and saturation parameters important for all applications.

Effects of nitroxides on the magnetic field and temperature dependence of 1/T1 of solvent water protons

“…Electron-proton interactions for systems of free radicals in weak hydrogen bond forming solvents have been studied with NMR methods by a number of groups (25)(26)(27)(28)(29)(59)(60)(61)(62)(63)(64). The data on temperature and/or magnetic field dependence of NMR relaxation rates for aqueous solutions of nitroxides are somewhat more limited.…”

“…Contributions due to scalar interaction between the unpaired electrons of nitroxide molecules and the protons of water have not been considered in the foregoing; dynamic polarization studies (26,(59)(60) and the near equality of T I and T2 at high magnetic fields (25,27) support the assumption that these interactions are negligible. Finally, the consequences of the simplifications of Freed's model theory have been investigated by Albrand et al (36), who compared spectral density functions based on experiment to those obtained from theory with more complex assumptions.…”

Three-Spin Effect in Dynamic Nuclear Polarization: Quantitative Determination of Internuclear Interactions

“…For bulk fluid samples without DPPH, relaxation times were measured by a spin-echo apparatus at 3700 G. Bulk relaxation times at this higher field are expected to be nearly the same as those at 74 G because they involve only the low-frequency nuclear transition q which is only 3% smaller at 3700 G for a reasonable correlation time of 15 This constancy of bulk fluid relaxation time with increasing field has been observed l6 for fluorobenzene at fields up to 133000 G. For bulk fluid samples without DPPH, relaxation times were measured by a spin-echo apparatus at 3700 G. Bulk relaxation times at this higher field are expected to be nearly the same as those at 74 G because they involve only the low-frequency nuclear transition q which is only 3% smaller at 3700 G for a reasonable correlation time of 15 This constancy of bulk fluid relaxation time with increasing field has been observed l6 for fluorobenzene at fields up to 133000 G.…”

Study of hydrogen bridge formation in free radical solutions via dynamic nuclear polarization

“…Dynamic nuclear polarization (DNP) of solvent nuclei in solutions of free radicals is sensitively dependent upon the time-dependence of the magnetic interaction with the radical electron spins. Normally, with increasing frequency and decreasing temperature a strong decrease in the maximum DNP-enhancement factor is observed [1][2][3][4][5][6]. This behaviour could, in a first approximation, be explained by dipole-dipole interactions which are modulated by the translational diffusion of individual molecules [1,3,7].…”