Abstract: The electron spin resonance spectra of N-carbamoyl-N-(l-oxyl-2,2,5,5-tetramethyl-pyrrolin-3-yl) urea compounds in ten solvents with different polarities were determined. It was found that the hyperfine splitting constant ANand spin density p_N. as well as the energy difference between two adjacent energy levels E of different pyrroline nitroxides in the same solvent increase with the increase of electron-withdrawing α-bility of the substituted groups on the aromatic ring. There was a linear correlation between A_N and Reichardt E_T value, and between A_N and Z value as well. Such a linear correlation was also found between solvent viscosity n and ESRrotational correlation time r. On the other hand, the correlation between Tand the relative height hr of peak on high field was reverse. The results are discussed in the paper.

Key words: Solvent effect, Ureas, Pyrroline nitroxides