Paramagnetic Relaxation of the Alkyl Radical

Inspection of the spectral shapes in Fig. 4 indicates that the linewidth of the six-line component due to the inner radical becomes sharpened with t2. This spectral sharpening is interpreted as being due to the dependence of the relaxation rate on the resonance frequency the relaxation is slower for the radicals at or near the peak of spectral lines than that for the radicals at the spectral tail because of the angular dependence of the hyperfine coupling. 

The longitudinal relaxation curves for the alkyl radicals monitored at the peak of the six-line spectrum (indicated by arrows in Fig. 6) are shown in Fig. 7. For n-alkanes, the relaxation rate decreases with the increase of the length of carbon chain, and it is independent of the radical concentration. This indicates that the cross relaxation between on-resonant and off-resonant radical spins is not the main source of the relaxation. In contrast, the relaxation rate for cyclohexane and polyethylene is the lowest, and depends on the radical concentration it decreases with the decrease of the radical concentration. This indicates that the cross relaxation contributes to the longitudinal relaxation in cyclohexane and polyethylene. 

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