Double modulation techniques in ESR and slow rotations of nitroxide spin labels

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Abstract
We have examined the potential of double modulation electron spin resonance in the studies of slow rotation and tumbling of nitroxide radicals. Contributions to the homogeneous broadening of electron spin packets, due to several distinct processes, could be analyzed separately. The system used throughout the present work was TANOL in glycerol–water matrix. In a radical concentration study at 77 K, we could analyze the homogeneous broadening due to electron spin flip–flop processes. It has been found that the rate of this process increases linearly with the radical concentration. The temperature dependence of spin packet linewidths has shown to be a good indicator of slow rotations and tumbling of nitroxide radicals. Assuming the jump diffusional model, we have determined that the rotational correlation time varied from 2×10−4 s at 150 K to about 4×10−5 s at 200 K. Electron dipolar interactions were found to contribute to the spin packet broadening when the tumbling rate was very low, while, at higher tumbling rates, these interactions were averaged out gradually. The broadening of spin packets was also found to depend on the water content in the matrix. OFF

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