ESR parameters and dynamic behavior of β-phosphorylated nitroxide radicals in glycerol solution

Abstract

The magnetic and dynamic properties of several β-phosphorylated nitroxide radicals in glycerol solution were studied by X-band ESR spectroscopy in the 180–380 K range. The parameters of interest were obtained from the least-squares fit of experimental spectra. These radicals display a large, weakly anisotropic 31P hyperfine coupling varying from 3 to 6 mT according to the orientation of the C(2)—P bond in the molecular frame and nearly independent of the temperature. Some of these radicals, with a five-membered cyclic structure, undergo an exchange between two conformers due to ring puckering, whose kinetics have been determined from the evolution of spectral shapes between 230 and 280 K. This motion, which modulates the 31P hyperfine coupling and leaves that of nitrogen unchanged, is hindered by substitution in position 4 of the ring of a proton by a phenyl group. All these radicals are found to undergo an anisotropic tumbling motion with correlation times decreasing from 100 to 0.1 ns between 280 and 380 K. © 1998 John Wiley & Sons, Ltd.