Molecular dynamics of nitroxides in glasses as studied by multi-frequency EPR

Pulsed multi-frequency EPR was used to investigate orientational molecular motion of the nitroxide spin probe (Fremy's salt) in glycerol glass near the glass transition temperature. By measuring echo-detected EPR spectra at different pulse separation times at resonance frequencies of 3, 9.5, 95 and 180 GHz, we were able to discriminate between different relaxation mechanisms and characterize the timescale of molecular reorientations (10(-7)-10(-10) s). We found that near the glass transition temperature, the orientation-dependent transverse relaxation is dominated by fast reorientational fluctuations, which may be overlapped with fast modulations of the canonical g-matrix values. The data was interpreted using a new simulation program for the orientation-dependent transverse relaxation rate 1/T2 of nitroxides based on different models for the molecular motion. The validity of the different models was assessed by comparing least-square fits of the simulated relaxation behaviour to the experimental data.