Showing papers by "Jack H. Freed published in 1983"

Quadrupole echo study of internal motions in polycrystalline media

Abstract: Etude theorique et experimentale du spectre d'echo quadripolaire de deux composes polycristallins deuteries: l'hexamethylbenzene-d 18 et l'iodure de trimethylsulfoxonium-d 9 . Les deux molecules presentent deux modes de mouvement interne independants

On cooperative modes of reorientation in liquid crystals

Abstract: The role of cooperative modes of reorientation in liquid crystals for the spectral densities of importance in magnetic resonance relaxation is examined from the viewpoint of microscopic models that can be solved analytically. These include linear arrays of rods in an applied external field whose orientations are strongly coupled and whose motion is described by overdamped and coupled diffusion equations. Also considered are Cayley‐tree models, which, at least for nearest neighbors, can mimic higher dimensions than one. All these models lead to the result that the qth normal mode of relaxation has a decay rate that depends upon q as sin2(qa/2), where a is the lattice spacing. Using simple scaling, it is argued that if a is replaced by a′≡La, so our model consists of blocks of L rodlike molecules with renormalized parameters, then molecular detail becomes less important and the model arrays become more physically representative. These results are then utilized to extend the standard hydrodynamic theory for ...

Anisotropic molecular motion of spin-labeled poly(methyl methacrylate) detected by ESR

Abstract: Anisotropic rotational motion of the side chain in poly(methy1 methacrylate) (PMMA) is inferred from the detailed analysis of the ESR spectrum of spin-labeled PMMA. Rotation about the axis parallel to the N-0 bond of the nitroxide label bonded at the end of the ester side chains was twice as fast as that about the axes perpendicular to this bond in the incipient motional narrowing region. This axially symmetric rotation is discussed in terms of the molecular motions of PMMA.

Multipulse sequences in electron‐spin echoes

Abstract: A method is described whereby Carr–Purcell (CP) multipulse sequences may be generated at a rate rapid enough for electron‐spin echoes ESE and in a manner consistent with constraints of typical ESE spectrometers. Both Carr–Purcell–Meiboom–Gill and alternating phase‐CP sequences may be produced. It is shown how such sequences may be utilized (1) to increase the rate of data collection, which is especially important in laser photo‐induced experiments, and (2) to study slow motions. They are respectively illustrated with results on transient spin‐polarized photoelectrons and electrons trapped in irradiated quartz.

Molecular orbital study of superoxide (O2-) adsorbed on titanium ions on oxide supports

Abstract: An extended Hueckel analysis of O/sub 2//sup -/ adsorbed on Ti ions on oxide supports is presented. The potential energy surface is obtained for deformations between the two limiting structures of the O/sub 2//sup -/ parallel to the surface and end-on. A stable conformation is found for the parallel form, consistent with recent ESR results, and there is a maximum in energy between the two limiting structures. The observed near equality of the ESR hyperfine splittings of the two O atoms is found to be consistent with a small tilting of the O/sub 2/ axis from the parallel conformation (i.e., < 10/sup 0/). A low barrier for planar rotation of O/sub 2//sup -/ about the axis perpendicular to the surface is also found, consistent with the small activation energy obtained from ESR line-shape analysis. The predicted g-tensor components allow unambiguous assignment of the ESR results, and a discussion is given on how variations in the extended Hueckel parameters improve agreement with experiment. 5 figures, 1 table.

Transverse Viscous Forces in Carr Walls and Possible Dynamic Consequences

Abstract: Expressions for the anisotropic viscous forces transverse to a nematic flow confined in a straight splay-bend wall are derived under simplifying assumptions. Some possible dynamic consequences, such as longitudinal splitting of the walls and movement of asymmetric walls in electro-hydrodynamic instabilities, are suggested. The relevance to recent experimental observations is discussed.

Electron-spin relaxation and ordering in smectic and supercooled nematic liquid crystals

Abstract: We report on careful line shape studies of slow motional and orientation dependent ESR spectra of a deuterated liquid‐crystal‐like spin probe dissolved in a benzilidene‐derivative (40,6) and in cyanobiphenyl derivative (S2 and 5CB) liquid crystals. The simulation of the ESR spectra is based on the Lanczos algorithm recently applied by Moro and Freed in a general and efficient formulation of slow motional and ordering effects on ESR line shapes. With 40,6 which exhibits monolayer smectic phases, we find that the main change in the spin relaxation upon passing from the nematic to the smectic A phase consists of changes occuring in ordering attributable to packing forces on functional groups. Such ordering effects appear to be further enhanced in the SB phase with consequent alterations in dynamics. With S2, which exhibits an interpenetrating bilayer smectic A phase, we find unusual ESR spectra in that phase which may be simulated on the basis of a model of cooperative distortions static on the ESR time scale, and superimposed on individual molecular reorientation. This mode is interpreted as a collective chain distortion which affects the orientational distribution of the piperidine ring of the spin probe. A similar phenomenon is observed in the supercooled nematic phase of 5CB, which is aligned by an electric field, and evidence is also found that the reorientational dynamics of this ring are affected by interaction with local cooperative modes in the liquid crystal (i.e., a SRLS mechanism previously proposed by Freed and co‐workers). Some microscopic characteristics of liquid crystals revealed by this and previous ESR spin probe studies are discussed.