Orientational ordering in uniaxial liquid crystals

Counsell, C.J.R.; Emsley, J. W.; Heaton, N.; Luckhurst, G. R.

doi:10.1080/00268978500103211

Cited by 79 publications

(39 citation statements)

References 12 publications

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“…3 Every laboratory appears to have its own favorite model. 3,[6][7][8][9][10][11][12][13][14][15][16][17][18] In practice, the predictions of the different models show a large degree of consistency. An earlier analysis of the NMR spectrum of orientationally ordered n-pentane utilized the fact that it is a magic solute and that size-and-shape models could provide a starting point for the spectral analysis.…”

Section: Introductionmentioning

confidence: 99%

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

Burnell

Weber

Dong

et al. 2015

The Journal of Chemical Physics

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The proton NMR spectra of n-pentane orientationally ordered in two nematic liquid-crystal solvents are studied over a wide temperature range and analysed using covariance matrix adaptation evolutionary strategy. Since alkanes possess small electrostatic moments, their anisotropic intermolecular interactions are dominated by short-range size-and-shape effects. As we assumed for n-butane, the anisotropic energy parameters of each n-pentane conformer are taken to be proportional to those of ethane and propane, independent of temperature. The observed temperature dependence of the n-pentane dipolar couplings allows a model-free separation between conformer degrees of order and conformer probabilities, which cannot be achieved at a single temperature. In this way for n-pentane 13 anisotropic energy parameters (two for trans trans, tt, five for trans gauche, tg, and three for each of gauche + gauche + , pp, and gauche + gauche − , pm), the isotropic trans-gauche energy difference E tg and its temperature coefficient E ′ tg are obtained. The value obtained for the extra energy associated with the proximity of the two methyl groups in the gauche + gauche − conformers (the pentane effect) is sensitive to minute details of other assumptions and is thus fixed in the calculations. Conformer populations are affected by the environment. In particular, anisotropic interactions increase the trans probability in the ordered phase. C 2015 AIP Publishing LLC. [http://dx

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Section: Introductionmentioning

confidence: 99%

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

Burnell

Weber

Dong

et al. 2015

The Journal of Chemical Physics

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“…In order to quantify the heating effect, we have measured the temperature dependence of the 13 C chemical shift from 300 K up to the nematic isotropic transition temperature (T NI ). These 8CB spectra were taken using a Bruker DSX-500 with a standard CP-MAS probe containing 4 mm o.d.…”

Section: Methodsmentioning

confidence: 99%

“…power used in standard nuclear magnetic resonance (NMR) experiments is able to produce heating effects in samples containing electrolytes or molecules with high permanent electric dipole moments. In particular, in liquid crystals of the 4-n-alkyl-4 0 -cyanobiphenyl series (electric dipole moment of approximately 14 ð 10 30 C m) 1 this heating effect has been observed when acquiring 13 C NMR spectra with high-power 1 H decoupling. There, the need for long time irradiation during the acquisition period, usually longer than 40 ms, with high r.f.…”

Section: Introductionmentioning

confidence: 95%

“…Thus, a phase lag is built up between the field and the dipoles, and the energy drawn from the electric source is dissipated as heat in the sample. Under this condition, the selection of the recycling time t rec between FIDs for a particular 13 C NMR experiment is not trivial. It is not enough to set t rec longer than four or five times T 1 (where T 1 is the spin-lattice relaxation time for carbon) as usual, but it is necessary to wait for the complete thermal relaxation of the system.…”

Section: Introductionmentioning

confidence: 97%

“…The sensitive parameter we used to observe the r.f. heating during the contact time was the chemical shift of each line in the 13 C spectrum. Then, we observed that during the contact time the lines shift in the direction of their positions in the isotropic phase.…”

Section: Introductionmentioning

confidence: 99%

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Radiofrequency‐induced temperature increase as a function of cross polarization contact time in 8CB

Chattah

Cucchietti

Hologne³

et al. 2002

Magnetic Reson in Chemistry

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In this work we analyse the non-negligible heating effect that can be produced by the radiofrequency during the contact time in Hartmann-Hahn cross polarization experiments in liquid crystals. We perform the analysis in the nematic phase of 4-n-octyl-4 -cyanobiphenyl (8CB). The heating effect is quantified through the chemical shifts of the carbon resonances, which move towards their respective positions in the isotropic phase when increasing the contact time. The calibration procedure involves an independent measurement of chemical shift dependence on externally controlled bath temperature. This variation was performed in the range between 300 and 313 K covering the smectic A and nematic mesophases. An overall heating rate of 0.16 K ms −1 is observed during the contact time in a cross polarization experiment performed at ! 1 /2p ' 80 kHz. This rate goes to approximately one-half for ! 1 /2p ' 60 kHz and becomes unobservable for ! 1 /2p < 50 kHz under typical experimental conditions.

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Dielectric Properties of Liquid Crystals Under High Pressure

Urban

Würflinger

1997

Advances in Chemical Physics

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Orientational ordering in uniaxial liquid crystals

Cited by 79 publications

References 12 publications

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

Radiofrequency‐induced temperature increase as a function of cross polarization contact time in 8CB

Dielectric Properties of Liquid Crystals Under High Pressure

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