250-GHZ EPR OF NITROXIDES IN THE SLOW-MOTIONAL REGIME - MODELS OF ROTATIONAL DIFFUSION

A 250-GHz electron paramagnetic resonance (EPR) study of the slow rota tional diffusion of two spin probes in toluene, viz., perdeuterated 2, 2,6,6-tetramethyl-4-piperidone (PDT) and 3-doxylcholestane (CSL) is pr esented. EPR spectra were obtained in the slow-motional and near-rigid limit regions, which corresponds to rotational correlation times 10(1 0) > tau(R) > 10(-6)s. These two probes differ significantly in size a nd shape, permitting a detailed exploration of the sensitivity of 250- GHz EPR to different aspects of the molecular dynamics such as rotatio nal anisotropy and non-Brownian diffusion. Nonlinear least-squares fit ting based on full stochastic Liouville calculations provides a sensit ive means for discriminating amongst motional models. PDT in toluene-d 8 is found to be well described by an approximate free diffusion model , whereas the larger spin probe, CSL, is best described by Brownian di ffusion. The slow-motional spectra at 250 GHz are most sensitive to th e diffusional model, the (geometric) mean diffusional rate, and axial diffusional anisotropy but less sensitive to rhombic deviations from a n axially symmetric diffusion tensor (i.e., to the general case R(x) n ot-equal R(y) not-equal R(z)). The slow-motional spectra of PDT were f it using anisotropic diffusion parameters determined from fast-motiona l spectra but are not very sensitive to such small anisotropies. For t he case of Brownian diffusion, CSL was best fit with N(y) = R(y)/(R(z) R(x))1/2 = 9.0 (where the y axis is the long axis of the molecule and x and z are perpendicular axes), which differs appreciably from the fa st-motional value of N(y) = 4.3 +/- 0.2 (and rho(x) = R(x)/R(z) = 0.5) . However, a mixed model of free-diffusional motion about the y axis w ith Brownian motion of this axis yields an N(y) close to the fast-moti onal value with comparable overall quality in fit compared to full Bro wnian motion. An important feature of the 250-GHz studies is the abili ty to measure very accurately the magnetic tensors needed for the moti onal studies. The theoretical modifications needed for inclusion of a fully anisotropic rotational diffusion tensor in the slow-motional EPR simulations are also given.