An EPR Study on Intramolecular Spin Exchange and Rotational Mobility of Nitroxides Linked by a Longchain

The effects of pressure and solvent on the intramolecular spin exchange of biradicals having two nitroxide fragments linked by a long flexible chain were studied by means of highpressure EPR technique. It was found that the intramolecular exchange interaction between nitroxides of biradical took place through the direct contact between them. By analyzing the observed EPR spectra, we have estimated the ratio(in/out) value of the average lifetime of the radical fragments inside a cage(in) to that outside the cage(out). The in/out values decreased with decreasing temperature and increasing pressure. The results suggest that the nearly cyclic conformation in a cage is favorable in solution. Further, the rotational correlation time of individual nitroxide was estimated from the anisotropic EPR signal, and the information on the segmental motion of the nitroxide group in biradical was obtained.