Highlights: 119Sn NMR tensors; 13C NMR of steroids; more from IUPAC on CS tensor conventions (will it ever end??); NMR of citrate in solids
Magnetic Resonance in Chemistry
Volume 46 Issue 7, Pages 690 - 692
Published Online: 28 Mar 2008
119Sn NMR chemical shift tensors in anhydrous and hydrated Si8O20(SnMe3)8 crystals
from Magnetic Resonance in Chemistry by Jian Jiao, Ming-Yung Lee, Craig E. Barnes, Edward W. Hagaman
119Sn chemical shift tensors of crystalline trialkyltin functionalized octameric spherosilicates, Si8O20(SnMe3)8, have been determined by fitting sideband intensities in solid-state magic angle spinning (MAS) NMR spectra. Tin chemical shift parameters are exquisitely sensitive to the presence of water of crystallization. Both hydrogen bonding and incipient oxygen-tin bonding from molecular water impact the local tin environment. Tin chemical shift tensors in the crystalline derivatives reflect the changes in geometry and coordination number at the tin centers. Chemical shift correlations on the crystalline derivatives, with known x-ray structures, are used to infer the tin coordination environment in an amorphous sample.
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Magnetic Resonance in Chemistry
Volume 46 Issue 8, Pages 718 - 725
13C solid-state NMR analysis of steroid compounds
by Jen-Hsien Yang, Yu Ho, Der-Lii M. Tzou
13C CP/MAS solid-state NMR spectroscopy has been utilized to analyze six steroid compounds, namely testosterone (Tes), hydrocortisone (Cor), trans-dehydroandrosterone (Adr), prednisolone (Prd), prednisone (Pre) and estradiol (Est). Among them, Tes displays a doublet pattern for all residues, whereas Prd, Pre and Est, exhibit exclusively singlets. For Cor and Adr, the 13C spectra contain both doublet and singlet patterns. The 13C doublet signal, with splittings of 0.2-1.5 ppm, are ascribed to local differences in the ring conformations associated with polymorphism. We have assigned all of the 13C resonances to the different residues in these steroid compounds on the basis of solution NMR data. The C-7, C-8, C-10, C-15 and C-16 residues of Tes, Cor and Adr consistently give rise to singlets or doublets with splittings of less than 0.5 ppm, indicating similar local conformations. Accompanying hydration and dehydration processes, a reversible phase transformation between [delta]- and [alpha]-crystal forms has been observed in Tes, corresponding to singlet and doublet 13C patterns, respectively. To further characterize the ring conformations in the [alpha]-form, we have successfully extracted chemical shift tensor elements for the 13C doublets. It is demonstrated that 13C solid-state NMR spectroscopy provides a reliable and sensitive means of characterizing polymorphism in steroids.
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Magnetic Resonance in Chemistry
Volume 46 Issue 6, Pages 582 - 598
Further Conventions for NMR Shielding and Chemical Shifts (IUPAC Recommendations 2008)
International Union of Pure and Applied Chemistry Physical and Biophysical Chemistry Division
Robin K. Harris 1 *, Edwin D. Becker 2, Sonia M. Cabral De Menezes 3, Pierre Granger 4, Roy E. Hoffman 5, Kurt W. Zilm 6,
Keywords
nuclear magnetic resonance • recommendations • chemical shifts • conventions • IUPAC Physical and Biophysical Chemistry Division • shielding tensors.
Abstract
IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS).
This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice. © 2008 IUPAC. Reprinted with permission from Pure Appl. Chem. 2008; 80: 59. This article can be freely downloaded from http://www.iupac.org/publications/pac/80/1/0059/ and can be copied, provided acknowledgement to IUPAC is given.
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Magnetic Resonance in Chemistry
Volume 46 Issue 5, Pages 408 - 417
Published Online: 28 Feb 2008
NMR spectroscopy of citrate in solids: cross-polarization kinetics in weakly coupled systems
from Magnetic Resonance in Chemistry by Jian Feng, Young J. Lee, James D. Kubicki, Richard J. Reeder, Brian L. Phillips
Solid-state NMR spectroscopy is a potentially powerful method for obtaining molecular level structural information crucial for understanding the specific relationship between calcite crystals and occluded organic molecules that are important in biomineralization and biomimetic materials. In this work, a method is developed based on cross-polarization/magic angle spinning (CP/MAS) NMR to measure the heteronuclear distances and obtain structural information for large intracrystalline citrate defects in a synthetic calcite/citrate composite. Using compounds with well-characterized crystal structures, Mg(II) citrate and Sr(II) citrate, a correlation is established between TIS, the CP time, and M2IS, the van Vleck heteronuclear dipolar second moment, which contains distance and structural information. This correlation is supported by peak assignments obtained from calculations of the 13C chemical shifts for crystalline Mg(II) citrate. On the basis of TIS-1 versus M2IS correlation, measurement of TIS for carbonate ions associated with citrate defects in a calcite(13C-enriched)/citrate coprecipitate yields an estimate for the distance between citrate and the nearest carbonate carbon that indicates close spatial proximity and provides useful constraints for future computational study. The applicability of TIS-1 versus M2IS correlations to other weakly coupled spin-1/2 systems is discussed in terms of the effects of 1H homonuclear dipolar coupling, using the CP kinetics of Zn(II) dihydroxybenzoate and kaolinite for comparison. The results suggest a limited range of correlation constants and indicate that quantitative information can be obtained from CP/MAS kinetics obtained under similar experimental conditions.
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