E. Bock

13C chemical shifts and spin‐lattice relaxation in the nematic liquid crystal 4‐cyano‐4′‐n‐pentylbiphenyl

The 13C chemical shifts and spin‐lattice relaxation times were obtained for all the resolved resonances in the isotropic and nematic phases of 5CB. NOE measurements were also obtained for the isotropic phase. A study of these data indicates that a flexibility gradient exists along the n‐pentyl chain and that D⊥, the rotational diffusion constant for the entire molecule, can be estimated. A study of the chemical shifts of 5CB shows that a single average shielding tensor is inadequate for a proper description of the chemical shifts of this liquid crystal molecule. These results are compared with those previously obtained for MBBA and PAA.

A deuterium NMR study of orientational order in tertiary-butyl bromide

Abstract An analysis of NMR lineshapes and spin-lattice relaxation times for the 2 H nucleus is reported for the liquid and solid phases I, II, and III of (CD 3 ) 3 CBr. The main results are a discontinuous increase of the rotational correlation time on melting (with a concomitant increase of the Arrhenius parameter) and, for phase II, a demonstration of the feasibility of measuring order parameters on the different timescales probed by the absorption profile and the relaxation experiment.

Carbon-13 Study of Relaxation and Chemical Shifts in the Smectic Liquid Crystal 8CB

Abstract The l3C chemical shifts and spin-lattice relaxation times were obtained for all the resolved resonances in the isotropic, nematic, and smectic phases of 8CB. NOE measurements were also obtained for the isotropic phase. A study of these data reveal some inconsistencies with dielectric and deuterium NMR studies, but general agreement with a 13C NMR study of 5CB. Discontinuities in relaxation times at both the nematic/isotropic and smectic/nematic phase transitions are observed for some of the carbon atoms. The difference between the components of the motionally averaged shielding tensor parallel and perpendicular to the long molecular axis is found for both the aromatic carbons and the first carbon of the octyl chain.

Molecular motions in smectic 4-n-pentyloxybenzylidened1-4′-n-heptylaniline: A nuclear relaxation study

Abstract Proton and deuteron spin-lattice relaxation time measurements are reported in the smectic liquid crystal 5O·7 − d 1 . The deuteron relaxation in the smectic phases is entirely attributable to molecular reorientation about the short molecular axis, while the proton relaxation is found to behave similarly to that of the smectic liquid crystal TBBA.

Deuterium NMR study of molecular order and conformation in the nematic phase of p‐azoxyanisole

A single orientation order parameter tensor is shown to be sufficient for the entire molecule in the nematic phase of perdeuterated p‐azoxyanisole by examining all the deuteron line splittings in the manner suggested by Bos and Doane. Two order parameters describing the molecular orientation order and the time averaged conformation of the molecule are measured. The relative conformation parameters are shown to be temperature independent within experimental error.

Carbon-13 Study of Relaxation in the 5CB Homologous Series

Abstract The carbon-13 chemical shifts and spin-lattice relaxation times were obtained for all the resolved resonances in the isotropic and nematic phases of 5CB, 6CB, and 7CB. NOE measurements were also obtained for the isotropic phases of 5CB, 6CB, 7CB, and 8CB. These results and those previously obtained by us for 8CB and for the isotropic phase of 5CB, demonstrate that even-odd effects occur for the carbon-13 spin-lattice relaxation times which cannot be explained by the differences in the nematic-isotropic transition temperatures. It is observed that the temperature dependence of the NOEs is significantly different for the protonated and unprotonated aromatic carbons.

Biological effects of magnetic fields: Chronic exposure of the nametode Panagrellus redivivus

The Panagrellus redivivus bioassay, an established monitor of adverse toxic effects of different environments, has been used to study the biological effects of exposure to static and time-varying uniform and gradient magnetic fields, and to time-varying magnetic field gradients superimposed on a static uniform magnetic field of 2.35 Tesla. Temporally stationary magnetic fields have no effect on the fitness of the test animals. Time-varying magnetic fields cause some inhibition of growth and maturation in the test populations. The combination of pulsed magnetic field gradients in a static uniform magnetic field also has a small detrimental effect on the fitness of the test animals.

A carbon-13 study of relaxation in the mesophases of 5O·7 and the 5CB homologous series

Abstract Carbon-13 spin-lattice relaxation times of the protonated ring carbons have been measured at 22·6 MHz in the nematic and all four smectic phases of 5O·7 (4-n-pentyloxybenzylidene-4′-n-heptylaniline). Dong has obtained the deuterium spectral densities J 1 and J 2 at 15·4MHz for the deuterated aniline ring of 5O·7-d 4, and has presented and applied a theory in which the spectral densities are expressed in terms of the diffusion constants D∥ and D⊊. His results are used to calculate 13C relaxation times from the spectral densities J 0, J 1 and J 2. The calculated 13C spin-lattice relaxation times are then compared with our experimental values to test the theory. The 13C spin-lattice relaxation times of all the resolved resonances in the various phases of the first four members of the 5CB homologous series have been published previously. Dong has also published an analysis of 5CB deuterium data, and we use his results for the diffusion constants D∥ and D⊊ to calculate 13C relaxation times of the prot...

Spectral Density of Motion in a Reentrant Nematic by Deuterium NMR

Abstract We report on a deuterium spin relaxation study of a 26 wt. % sample of perdeuterated hexyloxycyanobiphenyl (60CB-d 21) in octyloxycyanobiphenyl as a function of temperature at a Larmor frequency of 13.81 MHz. The binary mixture exhibits the following phase sequence: isotropic—nematic—smectic A—reentrant nematic—crystal. The DMR spectrum of this sample is relatively well-resolved and permits the measurement of the Zeeman (T 1Z) and quadrupole (T 1Q) spin-lattice relaxation times for several distinct deuterons in 60CB-d 21. The present study considers only the ring deuterons. In fact, there are two sets of ring deuterons and we restrict our study to the set with the smaller quadrupolar splitting to avoid spectral overlap from the methyl deuterons. These relaxation times were simultaneously measured by the phase-cycled Jeener-Broekaert sequence, and the spectral densities Jp (ρω) (p = 1,2) were found to increase continuously with decreasing temperature. Molecular reorientation seems to be a dominant...

DMR Study of Molecular Order In A Reentrant Nematic Liquid Crystal

Abstract We report on the measurement of molecular orientational order parameter Q of a perdeuterated 60CB molecule in a binary mixture of 60CB/80CB by means of deuterium quadrupolar splittings. The temperature variation of Q, both of the aromatic and the alkyl deuterons, behaves smoothly through the nematic, smectic A and reentrant nematic phases. These data are in contrast with the prediction of the molecular theory of McMillan-type and the phenomenological theory of Landau-type.