The Experts below are selected from a list of 63 Experts worldwide ranked by ideXlab platform
Cory M Widdifield - One of the best experts on this subject based on the ideXlab platform.
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calcium 43 chemical shift and electric field gradient tensor interplay a sensitive probe of structure polymorphism and hydRation
Physical Chemistry Chemical Physics, 2014Co-Authors: Cory M Widdifield, Igor L Moudrakovski, David L BryceAbstract:Calcium is the 5th most abundant element on earth, and is found in numerous biological tissues, proteins, materials, and increasingly in catalysts. However, due to a number of unfavourable nuclear properties, such as a low Magnetogyric Ratio, very low natural abundance, and its nuclear electric quadrupole moment, development of solid-state 43Ca NMR has been constrained relative to similar nuclides. In this study, 12 commonly-available calcium compounds are analyzed via43Ca solid-state NMR and the information which may be obtained by the measurement of both the 43Ca electric field gradient (EFG) and chemical shift tensors (the latter of which are extremely rare with only a handful of literature examples) is discussed. Combined with density functional theory (DFT) computations, this ‘tensor interplay’ is, for the first time for 43Ca, illustrated to be diagnostic in distinguishing polymorphs (e.g., calcium formate), and the degree of hydRation (e.g., CaCl2·2H2O and calcium tartrate tetrahydrate). For Ca(OH)2, we outline the first example of 1H to 43Ca cross-polarization on a sample at natural abundance in 43Ca. Using prior knowledge of the relationship between the isotropic calcium chemical shift and the calcium quadrupolar coupling constant (CQ) with coordination number, we postulate the coordination number in a sample of calcium levulinate dihydrate, which does not have a known crystal structure. Natural samples of CaCO3 (aragonite polymorph) are used to show that the synthetic structure is present in nature. Gauge-including projector augmented-wave (GIPAW) DFT computations using accepted crystal structures for many of these systems generally result in calculated NMR tensor parameters which are in very good agreement with the experimental observations. This combination of 43Ca NMR measurements with GIPAW DFT ultimately allows us to establish clear correlations between various solid-state 43Ca NMR observables and selected structural parameters, such as unit cell dimensions and average Ca–O bond distances.
Marcello Longeri - One of the best experts on this subject based on the ideXlab platform.
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the use of heteronuclear multiple quantum spectra in the automatic analysis of nmr spectra of samples dissolved in liquid crystalline phases
Liquid Crystals, 2001Co-Authors: Franca Castiglione, Giorgio Celebre, Giuseppina De Luca, Marcello LongeriAbstract:For spin systems with N I and N S 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 N I, (N I−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.
Gahan, Lawrence R. - One of the best experts on this subject based on the ideXlab platform.
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Embracing ligands. Synthesis, characterisation and the correlation between59Co NMR and ligand field parameters of Co(iii) complexes with a new class of nitrogen–thioether multidentate ligand
'Royal Society of Chemistry (RSC)', 2004Co-Authors: Sharrad, Clint A., Cavigliasso, Germán E., Stranger Robert, Gahan, Lawrence R.Abstract:The syntheses of the hexadentate ligands 2,2,10,10-tetra(methyleneamine)-4,8-dithiaundecane (PrNSamp), 2,2,11,11-tetra(methyleneamine)-4,9-dithiadodecane (BuNSamp), and 1,2-bis(4,4-methyleneamine)-2-thiapentyl)benzene (XyNSamp) are reported and the complexes [Co(RNSamp)] (R = Pr, Bu, Xy) characterised by single crystal X-ray study. The low-temperature (11 K) absorption spectra have been measured in Nafion films. From the observed positions of both spin-allowedA →T andA →T and spin forbiddenA →T andA →T bands, octahedral ligand-field parameters (10D , B and C) have been determined. DFT calculations suggest that significant interaction between the d–d and CT excitations occurs for the complexes. The calculations offer an explanation for the observed deviations from linearity of the relationship betweenCo Magnetogyric Ratio and β(ΔE) (β = the nephelauxetic Ratio; ΔE the energy of theA →T transition) for a series of amine and mixed amine/thioether donor complexes
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Embracing ligands. Synthesis, characterisation and the correlation between 59Co NMR and ligand field parameters of Co(III) complexes with a new class of nitrogen-thioether multidentate ligand
'Royal Society of Chemistry (RSC)', 2004Co-Authors: Sharrad, Clint A., Cavigliasso, Germán E., Stranger R., Gahan, Lawrence R.Abstract:The syntheses of the hexadentate ligands 2,2,10,10-tetra(methyleneamine)-4,8-dithiaundecane (PrN(4)S(2)amp), 2,2,11,11-tetra(methyleneamine)-4,9-dithiadodecane (BuN(4)S(2)amp), and 1,2-bis(4,4-methyleneamine)-2-thiapentyl)benzene (XyN(4)S(2)amp) are reported and the complexes [Co(RN(4)S(2)amp)](3+) (R = Pr, Bu, Xy) characterised by single crystal X-ray study. The low-temperature (11 K) absorption spectra have been measured in Nafion films. From the observed positions of both spin-allowed (1)A(1g) --> T-1(1g) and (1)A(1g) --> T-1(2g) and spin forbidden (1)A(1g) --> T-3(1g) and (1)A(1g) --> T-3(2g) bands, octahedral ligand-field parameters (10D(q), B and C) have been determined. DFT calculations suggest that significant interaction between the d-d and CT excitations occurs for the complexes. The calculations offer an explanation for the observed deviations from linearity of the relationship between Co-59 Magnetogyric Ratio and beta(DeltaE)(-1) (beta = the nephelauxetic Ratio; DeltaE the energy of the (1)A(1g) --> T-1(1g) transition) for a series of amine and mixed amine/thioether donor complexes
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Synthesis and characterization of cobalt(III) complexes with sinambic amplector ligands
'Elsevier BV', 2004Co-Authors: Sharrad, Clint A., Gahan, Lawrence R.Abstract:The synthesis of the hexadentate ligand 5,6-dimethyl-2,2,9,9-tetra(methyleneamine)-4,7-dithiadecane (1,2-Me(2)EtN(4)S(2)amp) is reported. The diastereiosomers were separated as cobalt(III) complexes using cation exchange chromatography. The rac and mesa isomers were characterized by NMR (C-13, H-1, Co-59), ESI-MS, UV-Vis spectroscopy and cyclic voltammetry. Single crystals of [Co(rac-1,2-Me(2)EtN(4)S(2)amp)] Cl-2(ClO4) (.) 2H(2)O were characterized by X-ray diffraction. The low-temperature (11 K) absorption spectra of the complexes have been measured in Nafion films and from the observed positions of both spin-allowed (1)A(1g) --> T-1(1g) and (1)A(1g) --> T-1(2g) and spin forbidden (1)A(1g) --> T-3(2g) bands, octahedral ligand-field parameters (10Dq, B and C) were determined. These results, in conjunction with the Co-59 NMR data, are used to further explore the relationship between the Co-59 Magnetogyric Ratio (gamma(Co)) and the product of the nephelauxetic Ratio and the wavelength of the (1)A(1g) --> T-1(1g) transition (beta(DeltaE)(-1)) for complexes of mixed donor nitrogen-thioether ligands. (C) 2004 Elsevier Ltd. All rights reserved
David L Bryce - One of the best experts on this subject based on the ideXlab platform.
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calcium 43 chemical shift and electric field gradient tensor interplay a sensitive probe of structure polymorphism and hydRation
Physical Chemistry Chemical Physics, 2014Co-Authors: Cory M Widdifield, Igor L Moudrakovski, David L BryceAbstract:Calcium is the 5th most abundant element on earth, and is found in numerous biological tissues, proteins, materials, and increasingly in catalysts. However, due to a number of unfavourable nuclear properties, such as a low Magnetogyric Ratio, very low natural abundance, and its nuclear electric quadrupole moment, development of solid-state 43Ca NMR has been constrained relative to similar nuclides. In this study, 12 commonly-available calcium compounds are analyzed via43Ca solid-state NMR and the information which may be obtained by the measurement of both the 43Ca electric field gradient (EFG) and chemical shift tensors (the latter of which are extremely rare with only a handful of literature examples) is discussed. Combined with density functional theory (DFT) computations, this ‘tensor interplay’ is, for the first time for 43Ca, illustrated to be diagnostic in distinguishing polymorphs (e.g., calcium formate), and the degree of hydRation (e.g., CaCl2·2H2O and calcium tartrate tetrahydrate). For Ca(OH)2, we outline the first example of 1H to 43Ca cross-polarization on a sample at natural abundance in 43Ca. Using prior knowledge of the relationship between the isotropic calcium chemical shift and the calcium quadrupolar coupling constant (CQ) with coordination number, we postulate the coordination number in a sample of calcium levulinate dihydrate, which does not have a known crystal structure. Natural samples of CaCO3 (aragonite polymorph) are used to show that the synthetic structure is present in nature. Gauge-including projector augmented-wave (GIPAW) DFT computations using accepted crystal structures for many of these systems generally result in calculated NMR tensor parameters which are in very good agreement with the experimental observations. This combination of 43Ca NMR measurements with GIPAW DFT ultimately allows us to establish clear correlations between various solid-state 43Ca NMR observables and selected structural parameters, such as unit cell dimensions and average Ca–O bond distances.
Mark E Smith - One of the best experts on this subject based on the ideXlab platform.
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Recent advances in solid-state 25mg NMR spectroscopy
Annual reports on NMR spectroscopy, 2012Co-Authors: Jair C. C. Freitas, Mark E SmithAbstract:Abstract In spite of the relatively unfavourable characteristics of the 25 Mg nuclide for NMR experiments—such as low natural abundance, small Magnetogyric Ratio and sizeable quadrupolar broadening—there is increasing evidence that solid-state 25 Mg NMR is a powerful tool for studies involving many different materials. Much of the recent boost in this field has been driven by instrumental improvements, such as the availability of high magnetic fields and fast-spinning speeds, as well as by the use of signal-enhancement methods developed in the past decade for half-integer spin quadrupolar nuclei. The most relevant advances in solid-state 25 Mg NMR spectroscopy are reviewed here, including a detailed account of the use of signal-enhancement methods and of the more recent applications of first-principles calculations of 25 Mg NMR parameters. Examples of the application of 25 Mg solid-state NMR are given for different classes of materials, including organic compounds, oxide-based materials, glasses, alloys and intermetallic compounds.