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6Li NMR Spectroscopy
The Experts below are selected from a list of 51 Experts worldwide ranked by ideXlab platform
David B Collum – One of the best experts on this subject based on the ideXlab platform.
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Lithium Enolates Derived from Weinreb Amides: Insights into Five-Membered Chelate Rings
The Journal of organic chemistry, 2016Co-Authors: Michael J. Houghton, David B CollumAbstract:Enolization of O-methyl hydroxamic acids (Weinreb amides) in tetrahydrofuran solution with lithium diisopropylamide affords predominantly tetrameric enolates. Aryl substituents on the enolates promote deaggregation. The aggregation states are assigned by using the method of continuous variation in conjunction with 6Li NMR Spectroscopy. Decoalescence of the tetramer resonance below −100 °C shows considerable spectral complexity attributed to isomerism of the methoxy-based chelates. Density funcfunctional theory calculations were used to examine the consequences of the bite angle of five-membered chelates in cubic tetramers and resulting solvation numbers that were higher than anticipated.
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Solution structures of lithium amino alkoxides used in highly enantioselective 1,2-additions.
Journal of the American Chemical Society, 2014Co-Authors: Angela M. Bruneau, Lara R Liou, David B CollumAbstract:Lithium ephedrates and norcarane-derived lithium amino alkoxides used to effect highly enantioselective 1,2-additions on large scales have been characterized in toluene and tetrahydrofuran. The method of continuous variations in conjunction with 6Li NMR Spectroscopy reveals that the lithium amino alkoxides are tetrameric. In each case, low-temperature 6Li NMR spectra show stereoisomerically pure homoaggregates displaying resonances consistent with an S4-symmetric cubic core rather than the alternative D2d core. These assignments are supported by density funcfunctional theory computations and conform to X-ray crystal structures. Slow aggregate exchanges are discussed in the context of amino alkoxides as chiral auxiliaries.
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structure determination using the method of continuous variation lithium phenolates solvated by protic and dipolar aprotic ligands
Journal of Organic Chemistry, 2013Co-Authors: Laura L Tomasevich, David B CollumAbstract:The method of continuous variation (MCV) was used in conjunction with 6Li NMR Spectroscopy to characterize four lithium phenolates solvated by a range of solvents, including N,N,N′,N′-tetramethylethylenediamine, Et2O, pyridine, protic amines, alcohols, and highly dipolar aprotic solvents. Dimers, trimers, and tetramers were observed, depending on the precise lithium phenolate–solvent combinations. Competition experiments (solvent swaps) provide insights into the relative propensities toward mixed solvation.
Gillian R. Goward – One of the best experts on this subject based on the ideXlab platform.
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An Improved Understanding of Li+ Hopping Pathways and Rates in Li3Fe2(PO4)3 Using Selective Inversion 6Li NMR Spectroscopy
The Journal of Physical Chemistry C, 2013Co-Authors: Danielle L. Smiley, L. J. M. Davis, Gillian R. GowardAbstract:6Li selective inversion NMR experiments are used to reveal Li ion exchange rates and energy barriers for Li ion hopping in monoclinic Li3Fe2(PO4)3. The three crystallographically unique Li sites in this material are well resolved by magic-angle spinning, thus allowing for the examination of all three exchange processes. We have revisited this material using selective inversion to probe dynamics, and energy barriers over the temperature range 268–397 K are found to be 0.37 ± 0.07, 0.53 ± 0.02, and 0.52 ± 0.03 eV for the three unique exchange pairs. The results presented here are consistent with the known Li3Fe2(PO4)3 crystal structure. The selective inversion experiment is more robust than 2D EXSY for the determination of energy barriers by NMR; this can be attributed to the efficiency of the one-dimensional technique, and an exchange model that accounts for multisite exchange and fast spin–lattice relaxation. Moreover, bond valence sum density maps provide a meaningful depiction of lithium ion diffusion p…
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an improved understanding of li hopping pathways and rates in li3fe2 po4 3 using selective inversion 6Li NMR Spectroscopy
Journal of Physical Chemistry C, 2013Co-Authors: Danielle L. Smiley, L. J. M. Davis, Gillian R. GowardAbstract:6Li selective inversion NMR experiments are used to reveal Li ion exchange rates and energy barriers for Li ion hopping in monoclinic Li3Fe2(PO4)3. The three crystallographically unique Li sites in this material are well resolved by magic-angle spinning, thus allowing for the examination of all three exchange processes. We have revisited this material using selective inversion to probe dynamics, and energy barriers over the temperature range 268–397 K are found to be 0.37 ± 0.07, 0.53 ± 0.02, and 0.52 ± 0.03 eV for the three unique exchange pairs. The results presented here are consistent with the known Li3Fe2(PO4)3 crystal structure. The selective inversion experiment is more robust than 2D EXSY for the determination of energy barriers by NMR; this can be attributed to the efficiency of the one-dimensional technique, and an exchange model that accounts for multisite exchange and fast spin–lattice relaxation. Moreover, bond valence sum density maps provide a meaningful depiction of lithium ion diffusion p…
Klaus Dr Bronstert – One of the best experts on this subject based on the ideXlab platform.
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reductive transformations 19 2 electron transfer induced dimerization of β alkylstyrenes and the structures of the resulting 1 4 dilithiobutanes 1
Chemische Berichte, 1991Co-Authors: Peter Schade, Thomas Schafer, Klaus Mullen, Dietmar Dr Bender, Konrad Knoll, Klaus Dr BronstertAbstract:Upon reaction with lithium β-alkylstyrenes undergo dimerization of the resulting radical anions with the formation of 2,3-dialkyl-1,4-dilithio-1,4-diphenyl butanes 1–8. The structure and the dynamic behavior of these dilithium organyls are investigated by cryoscopic measurements and 1H-, 13C-, and 6Li–NMR Spectroscopy at different temperatures. The butane chain adopts a cisoid conformation; the two benzyl subunits chelate one lithium cation while the other is solvent-separated from the hydrocarbon frame. Depending on the size of the alkyl substituents and the reaction temperature, the dimerization of the radical anions with the formation of two stereogenic centers can proceed diastereoselectively.
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Reductive Transformations, 19(2) Electron‐Transfer‐Induced Dimerization of β‐Alkylstyrenes and the Structures of the Resulting 1,4‐Dilithiobutanes(1)
Chemische Berichte, 1991Co-Authors: Peter Schade, Thomas Schafer, Klaus Mullen, Dietmar Dr Bender, Konrad Knoll, Klaus Dr BronstertAbstract:Upon reaction with lithium β-alkylstyrenes undergo dimerization of the resulting radical anions with the formation of 2,3-dialkyl-1,4-dilithio-1,4-diphenyl butanes 1–8. The structure and the dynamic behavior of these dilithium organyls are investigated by cryoscopic measurements and 1H-, 13C-, and 6Li–NMR Spectroscopy at different temperatures. The butane chain adopts a cisoid conformation; the two benzyl subunits chelate one lithium cation while the other is solvent-separated from the hydrocarbon frame. Depending on the size of the alkyl substituents and the reaction temperature, the dimerization of the radical anions with the formation of two stereogenic centers can proceed diastereoselectively.
Danielle L. Smiley – One of the best experts on this subject based on the ideXlab platform.
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An Improved Understanding of Li+ Hopping Pathways and Rates in Li3Fe2(PO4)3 Using Selective Inversion 6Li NMR Spectroscopy
The Journal of Physical Chemistry C, 2013Co-Authors: Danielle L. Smiley, L. J. M. Davis, Gillian R. GowardAbstract:6Li selective inversion NMR experiments are used to reveal Li ion exchange rates and energy barriers for Li ion hopping in monoclinic Li3Fe2(PO4)3. The three crystallographically unique Li sites in this material are well resolved by magic-angle spinning, thus allowing for the examination of all three exchange processes. We have revisited this material using selective inversion to probe dynamics, and energy barriers over the temperature range 268–397 K are found to be 0.37 ± 0.07, 0.53 ± 0.02, and 0.52 ± 0.03 eV for the three unique exchange pairs. The results presented here are consistent with the known Li3Fe2(PO4)3 crystal structure. The selective inversion experiment is more robust than 2D EXSY for the determination of energy barriers by NMR; this can be attributed to the efficiency of the one-dimensional technique, and an exchange model that accounts for multisite exchange and fast spin–lattice relaxation. Moreover, bond valence sum density maps provide a meaningful depiction of lithium ion diffusion p…
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an improved understanding of li hopping pathways and rates in li3fe2 po4 3 using selective inversion 6Li NMR Spectroscopy
Journal of Physical Chemistry C, 2013Co-Authors: Danielle L. Smiley, L. J. M. Davis, Gillian R. GowardAbstract:6Li selective inversion NMR experiments are used to reveal Li ion exchange rates and energy barriers for Li ion hopping in monoclinic Li3Fe2(PO4)3. The three crystallographically unique Li sites in this material are well resolved by magic-angle spinning, thus allowing for the examination of all three exchange processes. We have revisited this material using selective inversion to probe dynamics, and energy barriers over the temperature range 268–397 K are found to be 0.37 ± 0.07, 0.53 ± 0.02, and 0.52 ± 0.03 eV for the three unique exchange pairs. The results presented here are consistent with the known Li3Fe2(PO4)3 crystal structure. The selective inversion experiment is more robust than 2D EXSY for the determination of energy barriers by NMR; this can be attributed to the efficiency of the one-dimensional technique, and an exchange model that accounts for multisite exchange and fast spin–lattice relaxation. Moreover, bond valence sum density maps provide a meaningful depiction of lithium ion diffusion p…
Lara R Liou – One of the best experts on this subject based on the ideXlab platform.
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Solution structures of lithium amino alkoxides used in highly enantioselective 1,2-additions.
Journal of the American Chemical Society, 2014Co-Authors: Angela M. Bruneau, Lara R Liou, David B CollumAbstract:Lithium ephedrates and norcarane-derived lithium amino alkoxides used to effect highly enantioselective 1,2-additions on large scales have been characterized in toluene and tetrahydrofuran. The method of continuous variations in conjunction with 6Li NMR Spectroscopy reveals that the lithium amino alkoxides are tetrameric. In each case, low-temperature 6Li NMR spectra show stereoisomerically pure homoaggregates displaying resonances consistent with an S4-symmetric cubic core rather than the alternative D2d core. These assignments are supported by density functional theory computations and conform to X-ray crystal structures. Slow aggregate exchanges are discussed in the context of amino alkoxides as chiral auxiliaries.
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lithium phenolates solvated by tetrahydrofuran and 1 2 dimethoxyethane structure determination using the method of continuous variation
Journal of the American Chemical Society, 2009Co-Authors: Timothy S De Vries, Lara R Liou, Jocelyn M Gruver, Anandarup Goswami, Emily Jayne, David B CollumAbstract:The method of continuous variation in conjunction with 6Li NMR Spectroscopy was used to characterize lithium phenolates solvated by tetrahydrofuran and 1,2-dimethoxyethane. The strategy relies on the formation of ensembles of homo- and heteroaggregated phenolates. The symmetries and concentration dependencies of the heteroaggregates attest to the aggregation numbers of the homoaggregates. The structurally diverse phenols afford substrate- and solvent-dependent combinations of lithium phenolate monomers, dimers, trimers, tetramers, and pentamers. We discuss the refinement of protocols for characterizing O-lithiated species. Computational studies examine further the substituent and solvent dependencies of aggregation.
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structures of β amino ester enolates new strategies using the method of continuous variation
Journal of the American Chemical Society, 2008Co-Authors: Lara R Liou, Anne J Mcneil, Gilman E S Toombes, David B CollumAbstract:The solution structures of four enolates derived from β-amino esters are investigated using 6Li NMR Spectroscopy in conjunction with the method of continuous variation (method of Job). Ensembles of homo- and heteroaggregated enolates are generated by mixing enantiomers of a single enolate (R/S mixtures), opposite antipodes of two different enolates (R/S′ mixtures), and the same antipodes of two different enolates (R/R′ mixtures). The numbers of observable aggregates and their dependence on the mole fraction of the two enolates confirm the hexamer assignments. Inherent symmetries observable in the 6Li NMR spectra show the stereochemistry of chelation about the hexagonal drum.