ESR studies of anisotropic rotational reorientation and slow tumbling in liquid and frozen media. II. Saturation and nonsecular effects

Abstract

Careful studies are described of the ESR line shapes and saturation behavior for the peroxylamine disulfonate (PADS) radical dissolved in a range of glycerol‐water mixtures. This permitted studies where the rotational correlation time τ_R ranged from 3 × 10⁻¹² sec to < 10⁻⁶ sec. The unsaturated line shapes for 10⁻¹⁰ < τ_R < 10⁻⁹ sec are, as in Paper I, readily interpreted in terms of anisotropic rotational diffusion, but for τ_R < 10⁻¹⁰ sec anomalous behavior of the linewidths, which could be attributed to nonsecular spectral densities, occurs. Supporting experiments at 35 GHz, and also on ¹⁷O‐labeled PADS, are in excellent agreement with values of τ_R and N = 4.7 (the ratio of the two components of the axially symmetric diffusion tensor), and they supply further information on the anomalous behavior of the nonsecular spectral densities. A phenomenological treatment based on the memory function approach was found to qualitatively reproduce some observed features. The saturation studies were performed over both the motional narrowing and slow‐tumbling regions. The motional narrowing results could be analyzed in a straightforward manner to yield values of W_e, the electron‐spin flip rate, which are found to exhibit a weak dependence on τ_R, roughly as τ_R^−(1/4) for the glycerol solvents. The stochastic Liouville method is applied to an analysis of slow‐tumbling saturated spectra and generally reasonable agreement with experiment is achieved for the simplifying assumptions utilized. The rotationally invariant W_e obtained from the slow‐tumbling analyses are found to agree with the values of W_e extrapolated from the motional‐narrowing region. Other aspects of the slow‐tumbling saturation experiments and analysis are discussed.