ENDOR Relaxation Study on Phenyl Substituents

The proton ENDOR spectra of various phenyl substituted hydrocarbon radicals in solution show a marked temperature dependence of the positions, widths, and amplitudes of particular lines. These effects are analyzed in terms of three relaxation mechanisms: (i) internal hindered rotation and (ii) torsional oscillations of the phenyl rings, and (iii) molecular tumbling. From the temperature dependence of relaxation effects caused by mechanism (i) hindrance potentials for internal rotation of phenyl rings could be determined. The results are in good agreement with the predictions of simple j t M O theory. Relaxation effects caused by (ii) turned out to be negligible. Mechanism (iii) produces strong variations in linewidths at low temperatures due to the different hyperfine anisotropics depending on nuclear position.