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 dynam ic solvent cage. This formulation is appropriate for isotropic and ord ered fluids. The solvent cage is modeled in terms of a set of collecti ve variables that represent the instantaneous solvent structure around the probe and that reorient on a slower time scale than the probe. Th is ''slowly relaxing local structure'' model is incorporated into an a ugmented stochastic Liouville equation that is solved by efficient com putational means which enables nonlinear least squares fitting to expe rimental spectra. This formulation is applied to some recent slow moti onal ESR spectra obtained at 250 GHz. Such high-frequency ESR spectra have been shown to be particularly sensitive to the microscopic detail s of the molecular reorientational process. Significant improvements a re found in fitting the ESR spectra for the cases studied, viz., perde uterated 2,2,6,6-tetramethyl-4-piperidone (PDT) in toluene and 3-doxyl cholestane (CSL) in o-terphenyl (OTP), a glass-forming liquid, when co mpared 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 p robe itself, and it provides a potential for probe reorientation on th e order of 2-7 k(B)T. The cage potential for the PDT case is character ized by minima at more than one orientational angle, allowing for jump -type reorientations between such minima superimposed on substantial l ocal motions suggestive of earlier simulations based on a simple jump model. For CSL in OTP, weak negative ordering is found, consistent wit h an oblate-shaped local structure provided by the OTP solvent molecul es. These examples illustrate the potential of utilizing high-frequenc y slow motional ESR to discern details of solvent interactions associa ted with molecular reorientations in fluids.