The proton relaxation has been studied in methyl substituted aromatic radical ions in solution. The linewidth parameter T 2 of the protons in such radicals is goveméd by two intramolecular interactions, namely, the Fermi contact interaction and the anisotropic dipolar interaction. For the methyl protons the interactions are modulated in time by both intemal rotation and tumbling of the whole molecule in solution. Equations have been derived to account for the effect of these motions on the CH3 proton linewidth. The resulting formulas are used to analyze the NMR proton linewidths for the anion of 3, 3'-dimethylbiphenyl.

hexahydropyrene reveal an alternating line broadening, caused by the non-Synchronized motion of the two aliphatic bridges in the molecule. When this motion is described by the modified Bloch equations, using a foursites jump model, spectra are obtained which agree quite well with the experimental ones. T he potential barrier for the conformational interconversion can be calculated from the temperature dependence of the lifetime of each configuration and is found to be 10-0 ± 0-4 kcal mole-1 for the radical anion and 3-5 ± 0-4 kcal mole-1 for the radical cation. T h e inversion rate at 0°C equals 2-8 x 106 s-1 for the anion and 4-2 x 10® s-1 for the cation.