ESR study on rotational motions of some free radicals in the supercooled and glassy organic solvents

Abstract

Rotational motions of solute molecules in supercooled and glassy media were investigated with the temperature-dependent ESR spectra of the organic nitroxides and chlorine dioxide in some organic solvents. The rotational correlation times (τR’s) of the solutes were estimated by the method of Freed et al. for the nitroxides and by the spectral simulations based on the stochastic Liouville method for chlorine dioxide. From comparison between τR’s of the solutes and the dielectric relation times of the solvents or some equations which provide τR’s of molecules in supercooled liquid, it was concluded that a solute molecule rotates almost independently from motions of solvent molecules. The solvent-dependent parts of τR’s of the solute molecules obtained in the various combinations of the solute and solvents can be treated as perturbation for the rotational motion of a solute molecule. Nonlinear dependence of τR’s of the solutes on the relaxation times calculated from Stokes–Einstein’s relation shows such situation can be caused from inertial motion of a solute molecule.