Electron spin resonance studies of anisotropic ordering, spin relaxation, and slow tumbling in liquid crystalline solvents. 2

Abstract

A study is reported of the ESR line shapes in the slow-tumbling region for a cholestane (CSL) spin probe in nematic phase V solution. The line shapes are analyzed in terms of the Polnaszek, Bruno, Freed theory for spectra from slowly tumbling probes in ordered fluids. Rather good agreement with experiment is obtained from the slower motional spectra (rR > 5 X 10"9 s where rR is the rotational correlation time) utilizing a single term ordering potential. For these slow-motional spectra, effects of proton inhomogeneous broadening are relatively small. It is shown in this study on CSL that for rR > 10™9 s a motional-narrowing theory will lead to erroneous predictions for rR and ordering that become more serious as rR increases. The previous application of motional-narrowing theory by other workers to this probe is discussed in this light. The effects of various other factors upon the spectral simulations are discussed. In particular, it is shown that the spectra for this highly ordered probe should not be very sensitive to various aspects of model dependence of the reorientational motion as discussed in part I nor to effects from director fluctuations. Also the slower tumbling spectra are rather insensitive to anisotropy in tr (but the anisotropy may be determined from the faster motional spectra).Nevertheless, contributions from these effects could modify somewhat the values of rR and ordering that are obtained. The slow tumbling spectra are also insensitive to the angle of tilt of the nitroxide magnetic-tensor principal axis system in the x~y plane with respect to the molecular orientational axes. Our results show that the tilt angle of the magnetic z axis should not be very different from 0°.

Glass-like Behavior at Molecular Level Induced in a Nematic by a Dispersion of Aerosil Nanoparticles

“…in the cooling stage (not shown) were identical to those in the heating stage and are consistent with the values of D ? obtained for CSL in similar liquid crystal systems [7,19]. As in our previous work [5] we observe that the local rotational dynamics is only slightly affected by the presence of the aerosil even at a concentration of 10 wt%, where the macroscopic viscosity of the sample becomes very large (even at the highest temperature studied, the 10 wt% sample does not flow).…”

Slow-motion ESR of cholestane spin labels in planar multibilayers of diacyldigalactosyldiglycerides

“…-4.7 [19]. As the cholestane molecule is anchored with the oxazolidine ring in the headgroup region of the membrane we expect the ratio of the moments of inertia to increase.…”

Quantitation of lysolipids, fatty acids, and phospholipase A2 activity and correlation with membrane polarity

“…6A for example). In the case of slow motion spectrum such as in the gel phase of vesicles, an asymmetry is generally observed in the nitroxide spectrum ( 18 ). This asymmetry produces a shift of the MI = +1, Ϫ 1 lines toward the center, that is greater for high fi eld line (MI = Ϫ 1).…”

“…Thus changes around the neighborhood of spin probe in such rigid limit spectra are more clearly represented by changes in A Ϫ . The ESR line widths yield the rotational correlation time of the spin probe ( 16,18 ). This time depends on the viscosity of its environment referred to here as the microviscosity, , according to the Debye-Stokes-Einstein equation; πη τ 3 …”