Two-dimensional high-field electron spin echo spectroscopy of nitroxide spin probes

Relaxation of a highly asymmetric nitroxide spin probe in the frozen toluene solution was studied below `rigid limitʹ temperatures by means of two-dimensional high-field electron spin echo spectroscopy. It was shown that observed relaxation cannot be explained by reorientation motion of the nitroxide. A model is proposed in which an asymmetric relaxation is due to the modulation of the g-values caused by the fluctuations of the environment of the nitroxide. This model qualitatively agrees with the relaxation observed. The estimation of the correlation times of such modulation is done.