1,721,015 research outputs found
Internal Rotation in Alchyl Benzene derivatives: Maximum Entropy and Constrained Maximum Entropy analysis of the Dipolar Couplings of Samples dissolved in a Nematic Liquid Crystal solvent
No full textThe 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
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
CFD analysis of a homogenizing valve in presence of cavitation
No full textThe main target of this first study about an high pressure homogenizer valve is to describe the flow field within the homogenizing valve through CFD simulations considering pure water as operating fluid. The importance of simulating the cavitation is highlighted in the paper, where different approaches for the description of flow through the valve are considered. The results of simulations, in terms of pressure drop vs. volumetric flow rate characteristics, are compared with experimental data achieved from tests carried out on an industrial size homogenizer
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
An investigation of the structure and bond rotational potential of some fluorinated ethanes by NMR spectroscopy of solutions in nematic liquid crystalline solvents
No full textNMR spectra of 1,2-dibromo-1,1-difluoroethane and 1-bromo-2-iodo-tetrafluoroethane dissolved in nematic liquid crystalline solvents have been analysed to yield the magnitudes and signs of the scalar couplings, J(ij), and total anisotropic couplings, T-ij, between all the H-1, F-19, and C-13 nuclei, except for those between two C-13 nuclei. The values obtained for Tij in principle contain a contribution from J(ij)(aniso), the component along the static applied magnetic field of the anisotropic part of the electron-mediated spin-spin coupling. Neglecting this contribution allows partially averaged dipolar couplings, D-ij, to be extracted from the T-ij, and these were used to determine the structure, orientational order, and the conformational distribution generated by rotation about the C-C bond. The values obtained are compared with the results of calculations by ab initio and density functional methods. The differences found are no greater than those obtained for similar compounds which do not contain fluorine, so that there is no definitive evidence for significant contributions from J(CF)(aniso) or J(FF)(aniso) in the two compounds studied
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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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