It is shown that the E.S.R. spectra of long-chain nitroxide biradicals represent the superposition of the spectrum of the non-reacting radical moieties in the elongated conformation and of that of the cage where the radical moieties are close together and can interact with each other. Inside the cage a fast intramolecular motion is observed. Some thermodynamic parameters of the cage as well as thermodynamic parameters of the motion inside the cage are calculated from the experimental E.S.R. spectra of four biradicals.

X-band EPR studies of a series of undoped and carbon-doped titania powders reveal the presence of a carbon-centered radical species. The concentration of spins is five to six orders of magnitude smaller than the total carbon content. Photoexcitation of the carbon-doped samples increases the corresponding EPR signal intensities both under ultraviolet and visible light irradiation. After illumination, the original intensity is restored within a few hours. Samples with higher carbon radical concentration reveal higher photocatalytic activity in degradation of 4-chlorophenol under visible light irradiation.