1,720,982 research outputs found
NMR investigation of the dynamics of banana shaped molecules in the isotropic phase: a comparison with calamitic mesogens behaviour
No full textThe first translational self-diffusion NMR measurements in the isotropic phase of banana-shaped liquid crystals are reported. In this paper, two banana-shaped mesogens, having a similar molecular structure and showing a nematic phase, have been investigated by means of translational self-diffusion NMR, H-2 NMR spin-spin and H-1 NMR spin-lattice relaxation measurements in the isotropic phase. While H-1 diffusion and H-2 relaxation times reveal a peculiar slow dynamic behaviour of banana-shaped mesogens compared with calamitic mesogens, the H-1 relaxation times seem to be affected by fast dynamics only. The origin of these dynamic features is discussed in terms of overall and internal molecular motions, in the frame of recent speculations concerning the formation of molecular clusters or aggregates in the isotropic phase of banana-shaped liquid crystals
Traslational Self Diffusion anisotropy in the Smectic A phase measured by a static fringe field gradient 1H NMR diffusometry approach
No full textFrom the Synclinic to Anticlinic Smectic Phases: A Deuterium NMR and diffusion NMR study
No full textTranslational self diffusion in 4-n-octyloxy-4'cyanobiphenyl (8OCB) exploited with a static field gradient H-1 NMR diffusometry approach
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Atomistic computer simulation and experimental study on the dynamics of the n-cyanobiphenyls mesogenic series
No full textSeveral dynamic properties of the 4-n-alkyl-4'-cyanobiphenyls series (nCB) with n = 5, 6, 7, 8 have been studied by atomistic molecular dynamics (MD) simulations in the NVE ensemble adopting an ab initio derived force field (J. Phys. Chem. B 2007, 111, 2130). For each homologue, at least two state points, in the nematic and in the isotropic phase, as determined from lengthy equilibration runs performed in the previous work, have been considered. More than 10 ns have been produced at each state point, allowing us to calculate single-molecule properties as the translational and rotational diffusion coefficients along the series. An oscillating behavior of the diffusion coefficients, similar to the observed odd-even effect in static properties, has been predicted by MD. A good agreement with the results of purposely carried out (1)H NMR measurements is achieved, provided the MD values are increased by a factor that accounts for density overestimation. Spinning and tumbling rotational motions, monitored by calculating the rotational diffusion coefficients for all homologues in both phases, agree well with experimental data, at least for 5CB where NMR measures are reported. Collective properties, such as the isotropic shear viscosity and the rotational viscosity coefficient, have been computed for all homologues, and the MD values agree well with the experimental data reported in the literature. Finally, the origin of the odd-even effect, found for all the computed dynamic properties, is addressed
Synthesis and Properties of Achiral Asymmetric Dinuclear Tris(1-pyrazolyl)methane Complexes of Ru(II).
No full textExploiting the “complexes-as-metals/complexes-as-ligands” synthetic strategy [1] a family of six novel polypyridyl dinuclear species containing two different Ru tpm)(CL) units [tpm = tris(1-
pyrazolyl)methane, CL = 2,2’-bipyridine, 1,10-phenanthroline, 2,2’-biquinoline] bridged either by
4,4’-bipyridine or pyrazine (see the chart) have been prepared as hexafluorophosphate salts.
These compounds re-present the asymmetric analogues of six dinuclear complexes whose
synthesis and properties have been recently published. Notably, owing to the presence of a tripodal ligand, the metal centres are not chiral, thus the samples are not diastereomeric mixtures, a feature that prevents the structural characterization of oligonuclear trischelate complexes.
All novel complexes have been
thoroughly characterized by elemental analyses and IR spectroscopy, and their 1H
NMR spectra completely assigned on the basis of COSY and NOESY experiments.
The photophysical, electrochemical,
and spectroelectrochemical properties of the new species have also been studied
NMR studies of the ferroelectric SmC* phase
No full textIn this work an attempt is made to compare and rationalise the structural and dynamic behaviour of some ferroelectric rod-like mesogens studied by our group in recent years, mainly by means of 2H- and 13C-nuclear magnetic resonance (NMR) spectroscopy. This comparison concerning the local orientational order, the average molecular conformation and the chiral smectic C (SmC*) phase structure, with a brief overview on the reorientational dynamic properties, allowed us to identify some common behaviour of these ferroelectric rod-like mesogens when decreasing the temperature from lower ordered phases to and within the SmC* phase. The main results obtained by means of NMR studies on these mesogens are rationalised and discussed in comparison with detailed discrete Fourier transform computations of their molecular conformations (in the limit of isolated molecules) with the purpose of enlightening the role of intermolecular and packing interactions in the ferroelectric phase with respect to the smectic A and nematic phase
Dynamics of partially oriented L-phenylalanine-d8 in the CsPFO/H2O lyotropic system by 2H NMR relaxation studies
No full textDynamics of the L-phenylalanine-d(8) has been here investigated by analyzing the (2)H NMR spin-lattice relaxation times of this selectively deuterium enriched amino acid diluted in the cesium pentadecafluorooctanoate/water (CsPFO/ H(2)O) lyotropic system both in the nematic (N(D)(+)) and in the lamellar (L(D)) phases. Information on the internal and overall molecular motions as well as on collective motions has been achieved by a global fitting procedure. The dynamic processes affecting this probe molecule reflect its particular conformational and interaction properties with respect to the lyotropic environment. The best reproduction of the experimental data is obtained by assuming free internal reorientations of the benzylic moiety, which results in diffusion constants of the same order of magnitude of the overall molecular spinning motion. Moreover, the contribution of collective motions (order director fluctuations and layer undulations) is estimated to be greater than that commonly observed by other techniques in lyotropic systems
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