ESE-Detected Molecular Motions of Spin-Labeled Molecules on a Solid Inorganic Surface: Motional Models and Onset Temperatures

Electron spin echo (ESE) decays are highly sensitive to stochastic librations which are a general property of molecular solids of various origins. Adsorption of spin-labeled molecules on a solid inorganic surface under diluted conditions allows studying the motion devoid of the effects of cooperativity. Here, the temperature dependence of the motion-induced spin relaxation was studied for different types of spin-labeled molecules adsorbed on a SiO₂ surface. The spin relaxation rate for all the systems was found to attain well-defined maxima, which is in agreement with the model of uniaxial molecular librations. For spin-labeled stearic acid, the enhanced maximal relaxation rate was found which was interpreted as manifestation of two-axial (or planar) motion that is naturally expected for long flexible molecules. The data obtained suggest that the onset of the motions observed at two different temperatures, 100 K and 130 K, may be attributed to torsional and bending types of the motion, respectively. The models of non-cooperative motion developed for adsorbed molecules may become useful for analyzing motions in complex biological media, frozen ionic liquids, polymers, etc.