The Experts below are selected from a list of 81 Experts worldwide ranked by ideXlab platform
David L Bryce - One of the best experts on this subject based on the ideXlab platform.
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solid state 185 187re nmr and gipaw dft study of perrhenates and re2 co 10 Chemical Shift anisotropy nmr crystallography and a metal metal bond
Physical Chemistry Chemical Physics, 2015Co-Authors: Cory M Widdifield, Frederic A Perras, David L BryceAbstract:Advances in solid-state nuclear magnetic resonance (SSNMR) methods, such as dynamic nuclear polarization (DNP), intricate pulse sequences, and increased applied magnetic fields, allow for the study of systems which even very recently would be impractical. However, SSNMR methods using certain quadrupolar probe nuclei (i.e., I > 1/2), such as 185/187Re remain far from fully developed due to the exceedingly strong interaction between the quadrupole moment of these nuclei and local electric field gradients (EFGs). We present a detailed high-field (B0 = 21.1 T) experimental SSNMR study on several perrhenates (KReO4, AgReO4, Ca(ReO4)2·2H2O), as well as ReO3 and Re2(CO)10. We propose solid ReO3 as a new rhenium SSNMR Chemical Shift standard due to its reproducible and sharp 185/187Re NMR resonances. We show that for KReO4, previously poorly understood high-order quadrupole-induced effects (HOQIE) on the satellite transitions can be used to measure the EFG tensor asymmetry (i.e., ηQ) to nearly an order-of-magnitude greater precision than competing SSNMR and nuclear quadrupole resonance (NQR) approaches. Samples of AgReO4 and Ca(ReO4)2·2H2O enable us to comment on the effects of counter-ions and hydration upon Re(VII) Chemical Shifts. Calcium-43 and 185/187Re NMR tensor parameters allow us to conclude that two proposed crystal structures for Ca(ReO4)2·2H2O, which would be considered as distinct, are in fact the same structure. Study of Re2(CO)10 provides insights into the effects of Re–Re bonding on the rhenium NMR tensor parameters and rhenium oxidation state on the Re Chemical Shift Value. As overtone NQR experiments allowed us to precisely measure the 185/187Re EFG tensor of Re2(CO)10, we were able to measure rhenium Chemical Shift anisotropy (CSA) for the first time in a powdered sample. Experimental observations are supported by gauge-including projector augmented-wave (GIPAW) density functional theory (DFT) calculations, with NMR tensor calculations also provided for NH4ReO4, NaReO4 and RbReO4. These calculations are able to reproduce many of the experimental trends in rhenium δiso Values and EFG tensor magnitudes. Using KReO4 as a prototypical perrhenate-containing system, we establish a correlation between the tetrahedral shear strain parameter (|ψ|) and the nuclear electric quadrupolar coupling constant (CQ), which enables the refinement of the structure of ND4ReO4. Shortcomings in traditional DFT approaches, even when including relativistic effects via the zeroth-order regular approximation (ZORA), for calculating rhenium NMR tensor parameters are identified for Re2(CO)10.
L. Mafra - One of the best experts on this subject based on the ideXlab platform.
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Understanding the high catalytic activity of propylsulfonic acid-functionalized periodic mesoporous benzenesilicas by high-resolution H-1 solid-state NMR spectroscopy
Journal of Materials Chemistry, 2012Co-Authors: R. Siegel, E. Domingues, R. De Sousa, F. Jérome, C.m. Morais, N. Bion, P. Ferreira, L. MafraAbstract:Propylsulfonic acid-functionalized periodic mesoporous benzenesilica (Ph-PMO-SO3H, 1) has been shown to be exceptional solid catalysts in the acid-catalyzed condensation of indole on benzaldehyde. The reasons for this distinct behavior are so far not completely understood. Here, we present a study involving the combination of advanced high-resolution solid state magic-angle spinning (MAS) NMR characterization with the results of the application of hydrated and dehydrated 1 with different acid loadings in the acid-catalyzed condensation of indole on benzaldehyde attempting an explanation of the higher performance of these materials when compared with the conventional solid catalysts. H-1 MAS NMR investigations show the displacement of the propylsulfonic -SO3H protons to higher H-1 Chemical Shifts with increase of the sulfonic acid strength suggesting the formation of hydrogen bonds involving neighboring -SO3H groups. The acid strength of 1 is lowered by the presence of water. At low sulfonic acid loading the catalytic activity is surprisingly high and independent of the water presence. The 2D H-1-H-1 recoupling MAS NMR experiments indicate that the phenyl rings may protect the acidic sites against water solvation, thus affording a plausible explanation for the negligible effect of water on the catalytic activity of 1 with low acid loading. For a proton loading higher than 0.40 mmol g(-1), we observed a linear relationship between the catalyst TOF and the Chemical Shift Value of the -SO3H proton, thus showing that solid H-1 NMR appears to be a convenient tool to predict the catalytic activity of 1 in water.
Cory M Widdifield - One of the best experts on this subject based on the ideXlab platform.
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solid state 185 187re nmr and gipaw dft study of perrhenates and re2 co 10 Chemical Shift anisotropy nmr crystallography and a metal metal bond
Physical Chemistry Chemical Physics, 2015Co-Authors: Cory M Widdifield, Frederic A Perras, David L BryceAbstract:Advances in solid-state nuclear magnetic resonance (SSNMR) methods, such as dynamic nuclear polarization (DNP), intricate pulse sequences, and increased applied magnetic fields, allow for the study of systems which even very recently would be impractical. However, SSNMR methods using certain quadrupolar probe nuclei (i.e., I > 1/2), such as 185/187Re remain far from fully developed due to the exceedingly strong interaction between the quadrupole moment of these nuclei and local electric field gradients (EFGs). We present a detailed high-field (B0 = 21.1 T) experimental SSNMR study on several perrhenates (KReO4, AgReO4, Ca(ReO4)2·2H2O), as well as ReO3 and Re2(CO)10. We propose solid ReO3 as a new rhenium SSNMR Chemical Shift standard due to its reproducible and sharp 185/187Re NMR resonances. We show that for KReO4, previously poorly understood high-order quadrupole-induced effects (HOQIE) on the satellite transitions can be used to measure the EFG tensor asymmetry (i.e., ηQ) to nearly an order-of-magnitude greater precision than competing SSNMR and nuclear quadrupole resonance (NQR) approaches. Samples of AgReO4 and Ca(ReO4)2·2H2O enable us to comment on the effects of counter-ions and hydration upon Re(VII) Chemical Shifts. Calcium-43 and 185/187Re NMR tensor parameters allow us to conclude that two proposed crystal structures for Ca(ReO4)2·2H2O, which would be considered as distinct, are in fact the same structure. Study of Re2(CO)10 provides insights into the effects of Re–Re bonding on the rhenium NMR tensor parameters and rhenium oxidation state on the Re Chemical Shift Value. As overtone NQR experiments allowed us to precisely measure the 185/187Re EFG tensor of Re2(CO)10, we were able to measure rhenium Chemical Shift anisotropy (CSA) for the first time in a powdered sample. Experimental observations are supported by gauge-including projector augmented-wave (GIPAW) density functional theory (DFT) calculations, with NMR tensor calculations also provided for NH4ReO4, NaReO4 and RbReO4. These calculations are able to reproduce many of the experimental trends in rhenium δiso Values and EFG tensor magnitudes. Using KReO4 as a prototypical perrhenate-containing system, we establish a correlation between the tetrahedral shear strain parameter (|ψ|) and the nuclear electric quadrupolar coupling constant (CQ), which enables the refinement of the structure of ND4ReO4. Shortcomings in traditional DFT approaches, even when including relativistic effects via the zeroth-order regular approximation (ZORA), for calculating rhenium NMR tensor parameters are identified for Re2(CO)10.
Junya Mizutani - One of the best experts on this subject based on the ideXlab platform.
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1h nmr Chemical Shift Value of the isoflavone 5 hydroxyl proton as a convenient indicator of 6 substitution or 2 hydroxylation
Phytochemistry, 1991Co-Authors: Satoshi Tahara, John L Ingham, Fujinori Hanawa, Junya MizutaniAbstract:Abstract 1 H NMR examination of a range of simple and complex isoflavones has shown that the presence or absence of a 2′-hydroxyl group can be deduced from the Chemical Shift Value of the 5-hydroxyl proton determined in acetone- d 6 . The data can also be used to establish if a methoxyl group or an alkyl side structure is present at C-6, and distinguishes between prenylation at C-6 and C-8. The application of this 1 H NMR technique to the identification of Piscidia -derived isoflavones is discussed.
R. Siegel - One of the best experts on this subject based on the ideXlab platform.
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Understanding the high catalytic activity of propylsulfonic acid-functionalized periodic mesoporous benzenesilicas by high-resolution H-1 solid-state NMR spectroscopy
Journal of Materials Chemistry, 2012Co-Authors: R. Siegel, E. Domingues, R. De Sousa, F. Jérome, C.m. Morais, N. Bion, P. Ferreira, L. MafraAbstract:Propylsulfonic acid-functionalized periodic mesoporous benzenesilica (Ph-PMO-SO3H, 1) has been shown to be exceptional solid catalysts in the acid-catalyzed condensation of indole on benzaldehyde. The reasons for this distinct behavior are so far not completely understood. Here, we present a study involving the combination of advanced high-resolution solid state magic-angle spinning (MAS) NMR characterization with the results of the application of hydrated and dehydrated 1 with different acid loadings in the acid-catalyzed condensation of indole on benzaldehyde attempting an explanation of the higher performance of these materials when compared with the conventional solid catalysts. H-1 MAS NMR investigations show the displacement of the propylsulfonic -SO3H protons to higher H-1 Chemical Shifts with increase of the sulfonic acid strength suggesting the formation of hydrogen bonds involving neighboring -SO3H groups. The acid strength of 1 is lowered by the presence of water. At low sulfonic acid loading the catalytic activity is surprisingly high and independent of the water presence. The 2D H-1-H-1 recoupling MAS NMR experiments indicate that the phenyl rings may protect the acidic sites against water solvation, thus affording a plausible explanation for the negligible effect of water on the catalytic activity of 1 with low acid loading. For a proton loading higher than 0.40 mmol g(-1), we observed a linear relationship between the catalyst TOF and the Chemical Shift Value of the -SO3H proton, thus showing that solid H-1 NMR appears to be a convenient tool to predict the catalytic activity of 1 in water.