1,720,977 research outputs found
Short- and long-range contributions to the ordering of rigid planar solutes dissolved in a 55wt% ZLI1132+EBBA nematic mixture
No full textNine well studied (polar and non-polar) planar solutes have been used as probes to investigate the mechanism for orientational ordering in the nematic phase. Second-rank orientational order parameters have been derived from proton dipolar coupling constants, obtained from the NMR spectra of these molecules dissolved in a 55 wt% ZLI1132 + EBBA nematic mixture. The experimental data are interpreted in terms of an orienting potential experienced by molecules in a liquid crystal environment. A model for the orienting potential is presented which includes, in addition to short-range repulsion interactions described by the surface tensor model, long-range contributions treated according to a generalized reaction field approach. Least-squares fits of solvent dependent parameters to the experimental data have been performed to estimate the weight of the various contributions to the solute orientational order, and the results obtained in this way are discussed. Finally, the results obtained with the surface tensor model are compared with those from the size-shape potential suggested by other workers
Toward a generalized algorithm for the automated analysis of complex anisotropic NMR spectra
No full textAn existing algorithm, founded on the works of Stephenson and Binsch, for the automatic analysis of isotropic or simple anisotropic NMR spectra has been improved to treat very complex NMR spectra of molecules dissolved in nematic solvents. The main options added to the original algorithm are a wider choice of smoothing functions; the use of the principal component regression method; and the possibility of selecting molecular coordinates, order parameters, and spectral parameters as variables of the problem. By means of these new options, it has been possible to analyze automatically NMR spectra (even depending on 27 spectral parameters) of 16 molecules in an anisotropic environment. Details of each case are discussed
The use of heteronuclear multiple quantum spectra in the automatic analysis of NMR spectra of samples dissolved in liquid crystalline phases
No full textFor spin systems with NI and NS interacting nuclei of magnetogyric ratio γI and γS respectively, it is possible to obtain multiple quantum spectra characterized by changes in the magnetic numbers mX by NI, (NI-1)..., 0 with X = I, S. The use of such heteronuclear MQ spectra as an aid in the automatic analysis of NMR spectra of both rigid and flexible molecules dissolved in liquid crystalline phases, is discussed. It has been found that when I = 1H and S = 19F, the decoupling efficiency of the standard multipulse sequences, developed and optimized for heteronuclear decoupling in isotropic liquids, depends on the spin system and also on the overall orientation of the molecule when orienting mesophases are involved. These facts impose limitations on the applicability of a procedure which, when the decoupled heteronuclear MQ spectrum is obtained, significantly reduces the computing requirements of the automated spectral analysis
Toward a Generalized Algorithm for the Automated Analysis of Complex Anisotropic NMR Spectra
No full textAn existing algorithm, founded on the works of Stephenson and Binsch, for the automatic analysis of isotropic or simple anisotropic NMR spectra has been improved to treat very complex NMR spectra of molecules dissolved in nematic solvents. The main options added to the original algorithm are a wider choice of smoothing functions; the use of the principal component regression method; and the possibility of selecting molecular coordinates, order parameters, and spectral parameters as variables of the problem. By means of these new options, it has been possible to analyze automatically NMR spectra (even depending on 27 spectral parameters) of 16 molecules in an anisotropic environment. Details of each case are discussed. © 1998 Academic Press
The NMR Spectra of Samples Dissolved in Liquid-Crystalline Phases: Automatic Analysis with the Aid of Multiple Quantum Spectra - The Case of Flexible Molecules
No full textHomonuclear NS = 0 and heteronuclear NS ≠ 0 multiple quantum spectra, involving changes in the magnetic number mI by (NI-1), (NI-2), and (NI-3), with NI and NS the number of interacting nuclei of magnetogyric ratio γI and γS, are used for the automatic analysis of 1H NMR spectra of flexible molecules dissolved in liquid-crystalline phases. The automatic procedure has been applied to study molecules of general formula Ph-CH2-X starting from a parameter set having all the spectral parameters set to zero. The results of such an analysis are then used as starting parameters for analysis of the single quantum spectrum. The method was first tested when X = Br and X = H in order to compare strategies differing for the types of parameters used and was then applied to the analysis of 3-phenylprop-1-yne. © 2000 Academic Press
The NMR spectra of samples dissolved in liquid-crystalline phases. automatic analysis with the aid of multiple-quantum spectra
No full textA new strategy is described for analyzing the NMR spectra of molecules dissolved in liquid-crystalline phases. The multiplequantum spectra which involve a change in the magnetic quantum number m by N - 1, where N is the maximum possible, are recorded and analyzed automatically starting from a parameter set having all the spectral parameters set to zero. The results of such an analysis are then used as the starting parameters in an analysis of the single-quantum spectrum using an automatic analysis procedure. The method is illustrated by analyses of the proton spectra of bromobenzene (five spins) and ethynylbenzene and naphthaquinone (six spins) dissolved in nematic solvents. O 1996 Academic Press, Inc
Multitechnique investigation of conformational features of small molecules: The case of methyl phenyl sulfoxide
No full textThe conformational distribution of methyl phenyl sulfoxide (a molecule representative of a very important
class of reagents widely used in asymmetric synthesis) has been studied in two different phases of matter
(gas phase and solution) by a comprehensive approach including theoretical calculations, microwave
spectroscopy, liquid crystal NMR experiments, and atomistic molecular dynamics computer simulations. The
aim was to investigate the combined action of intra- and intermolecular interactions in determining the
molecule’s conformational equilibrium, upon which important physicochemical properties (inter alia, the
chemoselectivity) significantly depend. Basically, the results converge in describing the tendency of the molecule
to favor stable conformations governed by intramolecular interactions (in particular, the expected optimization
between steric repulsion and conjugation of ð systems). However, significant solvent effects (whose “absolute”
magnitude is actually difficult to assess, due to a certain “method-dependence” of the results) have been also
detected
The structure of ethylbenzene as a solute in liquid crystalline solvents via analysis of proton NMR spectra
No full textPrevious attempts to analyze the proton spectrum of ethylbenzene as a solute in nematic liquid crystalline solvents failed, but a successful strategy has now been devised and is described here. The proton spectra of samples of ethylbenzene dissolved in four different liquid crystals have been analyzed to yield sets of the partially-averaged dipolar couplings, D-ij. The couplings are then used to test models for the structure and conformation of this molecule
A combined LX-NMR and molecular dynamics investigation of the bulk and local structure of ionic liquid crystals
No full textThe unique power of NMR spectroscopy in anisotropic media (LX-NMR) as a tool to obtain local and bulk structural information, combined with the effectiveness of molecular dynamics simulations at the atomistic level, shows very attractive potentialities for the study of interesting, even though still poorly understood, materials such as Ionic Liquid Crystals (ILCs). In this work, we focused our attention, in particular, on the orientational ordering of two mesophases: 1-dodecyl-3-methylimidazolium chloride, [C12C1im]Cl, and 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C12C1im][BF4]. Both ILCs were studied by a 2H NMR direct investigation of the molecules forming the phases, suitably deuterated, and by 1H NMR spectroscopy, using the small rigid probe-solutes 1,4-dichlorobenzene (DCB), dissolved in [C12C1im][BF4] and [C12C1im]Cl, and 1,4-dibromobenzene (DBB) dissolved in [C12C1im][BF4], to probe the local, internal structure and organization of the mesophases. The experimental results were then compared with the predictions, by atomistic MD simulations, of the structure of the smectic phase of the two salts, at two selected temperatures, containing a single DCB molecule as a probe. The MD simulations show that the DCB solute is distributed only within the hydrophobic layers of the ILC. Orientational order parameters of the imidazolium cations and of the DCB molecule were obtained and compared with the experiments, showing a general good agreement and allowing a deeper understanding of the microscopic structure of the systems
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