Roy E. Hoffman

Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008)

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 parts 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 is reviewed in some detail, and recommendations are given for best practice.

Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008).

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).

Further conventions for NMR shielding and chemical shifts IUPAC recommendations 2008.

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 (1)H 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 (13)C 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.

High-resolution NMR "chromatography" using a liquids spectrometer.

NMR spectroscopy is an excellent tool for the structural analysis of pure compounds. However, for mixtures it performs poorly because of overlapping signals. Diffusion can be used to separate compounds of widely differing molecular weight but the amount of separation is usually insufficient. Addition of a solid medium, analogous to the stationary phase in chromatography, can preferentially slow the diffusion of some components of a mixture permitting separation in the diffusion dimension. However, this would usually require a solid-state NMR spectrometer otherwise the signals would be too broad. Susceptibility matching the solvent to the solid medium yields a spectrum with narrow signals allowing the measurement of a DOSY spectrum with enhanced separation in the diffusion dimension.

Transitions and loci of solubilization of nutraceuticals in U-type nonionic microemulsions studied by self-diffusion NMR

U-type microemulsions based on five food-grade ingredients, water, R(+)-limonene, ethanol, propylene glycol, (or glycerol) and ethoxylated sorbitan esters (Tween 60 or Tween 80) were studied. The U-type phase diagram is characterized by a unique composition, fully dilutable with the aqueous phase that inverts progressively from an L2 phase to an L1 phase via a bicontinuous structure without phase separation. The ‘oil concentrate’ (surfactant, oil, alcohol) is loaded with water-insoluble solubilizates (phytosterols, lutein and lycopene) a few times more than the solubility capacity of the oil phase (without the surfactant). The maximum solubilization capacity (μ-value) was obtained for phytosterols and the minimum solubilization capacity was for lycopene. All solubilization values decrease with aqueous phase dilution. If solubilization is calculated on the basis of the oil content (α-value) or the oil + surfactant (γ-value) it is obvious that the interface plays a key role in the solubilization.The lipophilic solubilizates (by SD-NMR) are tightly packed and well accommodated at interfaces that convex (hydrophobic-in-nature) toward the water (water-in-oil microemulsions). Solubilization at the bicontinuous interface is lower and the solubilizates are loose packed. Once phase inversion occurs, and the interface becomes more hydrophilic and transforms into oil-in-water microemulsion, the solubilization becomes minimal, and most of the solubilizate desorbs. Phytosterols and lycopene induce the transition from W/O to bicontinuous and it occurs at lower water content (ca. 25 wt% aqueous phase in the presence of solubilizate vs. 35 wt% in its absence). The transition from bicontinuous to O/W is mostly unaffected (or undetected) since the interface flattens out and the solubilizate does not affect the curvature much. Lutein displays different behavior, the transition, from bicontinuous phase to O/W, occurs at higher water contents because its adsorption and packing are significantly tighter. Solubilization capacities of each of the nutraceuticals were determined for all dilution compositions.

Variations on the chemical shift of TMS

The chemical shift of TMS is commonly assumed to be zero. However, it varies by over 1 ppm for 1H and 4 ppm for 13C and shows a correlation with the physical properties of the solvent. Using the commonly accepted convention that TMS always resonates at zero leads to significant errors when comparing chemical shifts in different solvents. A new method for measuring absolute chemical shift with a conventional NMR spectrometer is demonstrated. The observed chemical shift is corrected by measuring and correcting for susceptibility and shape factor. Practical suggestions are made for modifying the current chemical shift standard while maintaining compatibility with earlier literature.

Effect of Sodium Diclofenac Loads on Mesophase Components and Structure

We studied the effect of a model electrolytic drug on intermolecular interactions, conformational changes, and phase transitions in structured discontinuous cubic QL lyotropic liquid crystals. These changes were due to competition with hydration of the lipid headgroups. Structural changes of the phase induced by solubilization loads of sodium diclofenac (Na-DFC) were investigated by directly observing the water, ethanol, and Na-DFC components of the resulting phases using 2H and 23Na NMR. Na-DFC interacted with the surfactant glycerol monoolein (GMO) at the interface while interfering with the mesophase curvature and also competed with hydration of the surfactant headgroups. Increasing quantities of solubilized Na-DFC promoted phase transitions from cubic phase (discontinuous (QL) and bicontinuous (Q)) into lamellar structures and subsequently into a disordered lamellar phase. Quadrupolar coupling of deuterated ethanol by 2H NMR showed that it is located near the headgroups of the lipid and apparently is hydrogen bonded to the GMO headgroups. A phase transition between two lamellar phases (L alpha to L alpha*) was seen by 23Na NMR of Na-DFC at a concentration where the characteristics of the drug change from kosmotropic to chaotropic. These findings show that loads of solubilized drug may affect the structure of its vehicle and, as a result, its transport across skin-blood barriers. The structural changes of the mesophase may also aid controlled drug delivery.

π-Conjugated Benzoperylenes: Sequential C–S Bond Cleavage and Charge Distribution Patterns of the Anions

Chemical reduction of polyaromatic hydrocarbons yielded solutions of long-lived polyanionic species. Reduction of a sulfur heterocycle afforded a stable sulfur-containing dianion. Sulfur extrusion from this dianion proceeded upon further contact with the reducing metal. NMR and UV studies indicate a sulfur extrusion mechanism different than that previously observed in THF. Electron transfer to the already reduced hydrocarbon skeleton results in the stepwise cleavage of the two C–S bonds and the extrusion of a sulfur atom. Dimers of aromatic hydrocarbons such as pyrene and phenanthrene have been reduced as well. The interplay between coulombic repulsions and resonance energies is described.

NMR spectroscopic study of the Murex trunculus dyeing process

It is widely accepted that indigo dyes derived from Murex trunculus were used to produce the biblical dyes tekhelet and argaman. We describe a method of following the debromination of natural leucoindigos and their binding to wool using NMR spectroscopy. Debromination is observed prior to reaction with the wool and prior to oxidation. Binding to the wool is shown to occur prior to oxidation. NMR allows the dyeing process to be followed. This, in principle, could be used to correct problems during dyeing that would increase the reliability of the process. Copyright

Nonstoichiometric gelation of cyclodextrins and included planar guests.

A generic family of low molecular weight binary gels comprising beta-cyclodextrin (beta-CD) and one of a large variety of polyaromatic hydrocarbons (PAHs) in dimethylformamide (DMF), pyridine, and other polar solvents is described. The system is rather general and robust. It tolerates large changes in each of the major ingredients without losing gelation ability. alpha- and gamma-CD, and negatively or positively modified beta-CD (e.g., sulfate-, phosphate-, or amine-tethered beta-CD) as well as methylated beta-CD are all effective gelators. The cogelators encompass a similarly large variety of compounds characterized by the ability to form an ovular inclusion complex with the CD molecules and a capability to stack outside the CD cap to give long-range order far from the CD cap. Despite the low ratio between the CD and the cogelators, we show that most of the CD molecules are retained in the liquid phase and do not participate directly in the actual construction of the gel network. In fact, most of the sulfated and phosphated beta-CDs can be cleaned off the gel structure by electrophoresis, leaving an intact gel porous structure. The nonstoichiometric nature of the gel is underscored by the fact that one molecule of beta-CD can combine with as few as three molecules of chrysene or as many as 450 molecules of chrysene to gelate an additional 35,000-40,000 molecules of the solvent.