SLOW MOTIONAL ESR IN COMPLEX FLUIDS - THE SLOWLY RELAXING LOCAL-STRUCTURE MODEL OF SOLVENT CAGE EFFECTS

A detailed formulation is presented for the analysis of slow motional ESR in terms of the reorientation of the probe molecule within a dynamic solvent cage. This formulation is appropriate for isotropic and ordered fluids. The solvent cage is modeled in terms of a set of collective variables that represent the instantaneous solvent structure around the probe and that reorient on a slower time scale than the probe. This ''slowly relaxing local structure'' model is incorporated into an augmented stochastic Liouville equation that is solved by efficient computational means which enables nonlinear least squares fitting to experimental spectra. This formulation is applied to some recent slow motional ESR spectra obtained at 250 GHz. Such high-frequency ESR spectra have been shown to be particularly sensitive to the microscopic details of the molecular reorientational process. Significant improvements are found in fitting the ESR spectra for the cases studied, viz., perdeuterated 2,2,6,6-tetramethyl-4-piperidone (PDT) in toluene and 3-doxylcholestane (CSL) in o-terphenyl (OTP), a glass-forming liquid, when compared to a model of simple Brownian reorientation. In both cases the cage is found to relax at least 1 order of magnitude slower than the probe itself, and it provides a potential for probe reorientation on the order of 2-7 k(B)T. The cage potential for the PDT case is characterized by minima at more than one orientational angle, allowing for jump-type reorientations between such minima superimposed on substantial local motions suggestive of earlier simulations based on a simple jump model. For CSL in OTP, weak negative ordering is found, consistent with an oblate-shaped local structure provided by the OTP solvent molecules. These examples illustrate the potential of utilizing high-frequency slow motional ESR to discern details of solvent interactions associated with molecular reorientations in fluids.