Nuclear Relaxation in Molecular Liquids Containing Free Radicals

Publisher Website
Google Scholar
Abstract
Radio‐frequency pulse methods have been used to investigate the solvent‐proton and fluorine nuclear relaxation mechanisms in solutions of the organic free radicals α,α′‐diphenyl‐β‐picrylhydrazyl (DPPH) and of di‐tertiary‐butylnitroxide (DTBN) in molecular liquids. The protons 1/T 1 and 1/T 2 depend linearly upon DPPH concentration in benzene, as predicted for relaxation governed by nucleus—free‐radical interactions. The protonT 1/T 2 ratio is close to unity (∼1.2), at 28 Mc/sec and 21°C, for six different solvents containing DPPH, and eight with DTBN, indicating the predominance of nucleus—electron, dipole—dipole relaxation. The fluorine T 1/T 2 ratio is about 1.7 for DPPH and 1.5 for DTBN solutions in 1,3,5‐trifluorobenzene and in hexafluorobenzene, indicating an appreciable hyperfine interaction between solvent fluorines and the free‐radical odd electron. The temperature dependences of the proton and fluorine relaxation in these solutions, and in benzene solutions of DPPH and of DTBN correspond to apparent activation energies of about 2 kcal/mole for the motions governing the relaxation. The frequency dependences, between 3.74 and 36 Mc/sec at 21°C, of the nuclear relaxation in the solutions do not agree with relaxation characterized by only one correlation time. It is proposed that the free radicals are but weakly solvated and that the relaxation results from a combination of translational and rotational motions and, for fluorine, the A I·S interaction.