13C INEPT diffusion-ordered NMR spectroscopy (DOSY) with internal references

13C INEPT Diffusion-ordered NMR spectroscopy (DOSY) with an internal reference system was developed to study the aggregation state of THF-solvated LDA dimeric complex. Six components are clearly identified in the diffusion dimension, and their DOSY-generated 13C INEPT spectrum slices agree extremely well with their respective INEPT spectra. The correlation between log D and log FW of the linear least-squares fit to reference points of all components is exceptionally high: (r = 0.9985).     

Silicon-29 NMR spectroscopy in carbohydrate chemistry: Galactose derivatives

²⁹Si NMR spectra of the O-trimethylsilyl (OTMS) derivatives of various methyl α- and β-D -galactopyranosides have been recorded. The effect of changes in the anomeric configuration provides a means of assigning the resonance of the 2-OTMS substituent. Whereas the signal of the OTMS group attached at the 6-position can be assigned readily, those of the OTMS group at the 3- or 4-position cannot be assigned unequivocally.     

Tools and strategies for processing diffusion-ordered 2D NMR spectroscopy (DOSY) of a broad,featureless resonance: an application to methylaluminoxane (MAO)

DOSY has been extremely successful in many studies of molecular weight distributions, especially when the components are separable along the chemical shift axis. However, an unresolved NMR resonance yields the familiar problem of overlapping exponential decays. In a study of methylaluminoxane (MAO), a set of data processing and simulation tools were developed: read Bruker data files (Matlab); preliminary non-linear least-squares fit with f-test (Matlab); movie generation of the fits (Matlab); conversion of diffusion coefficients to molecular masses through molecular volumes (Gaussian-98); and simulation of DOSY data sets for various molecular mass distributions (Mathematica). These tools are presented here and briefly compared with other DOSY analysis methods.Electronic Supplementary Material Supplementary material is available in the online version of this article at . The following Matlab and Mathematica files are made available: Plot_Raw_DOSY.m, Fit_Two_Component.m (which calls One_Gaussian_LEASTSQ.m, Two_Gaussian_LEASTSQ.m, and pFTest.m), and DOSY_theory.nb. Note: Matlab v5.2 optimization toolbox, as supplied, lacked a confidence interval subroutine; upon our request, confint.m was provided to the authors by Mathworks, Inc. Newer versions of Matlab have a similar program already included in the optimization     

Natural products dereplication by diffusion ordered NMR spectroscopy (DOSY)

Diffusion-ordered NMR spectroscopy (DOSY) can be used to analyze mixtures of compounds since resonances deriving from different compounds are distinguished by their diffusion coefficients (D). Previously, DOSY has mostly been used for organometallic and polymer analysis, we have now applied DOSY to investigate diffusion coefficients of structurally diverse organic compounds such as natural products (NP). The experimental Ds derived from 55 diverse NPs has allowed us to establish a power law relationship between D and molecular weight (MW) and therefore predict MW from experimental D. We have shown that D is also affected by factors such as hydrogen bonding, molar density and molecular shape of the compound and we have generated new models that incorporate experimentally derived variables for these factors so that more accurate predictions of MW can be calculated from experimental D. The recognition that multiple physicochemical properties affect D has allowed us to generate a polynomial equation based on multiple linear regression analysis of eight calculated physicochemical properties from 63 compounds to accurately correlate predicted D with experimental D for any known organic compound. This equation has been used to calculate predicted D for 217 043 compounds present in a publicly available natural product database (DEREP-NP) and to dereplicate known NPs in a mixture based on matching of experimental D and structural features derived from NMR analysis with predicted D and calculated structural features in the database. These models have been validated by the dereplication of a mixture of two known sesquiterpenes obtained from Tasmannia xerophila and the identification of new alkaloids from the bryozoan Amathia lamourouxi. These new methodologies allow the MW of compounds in mixtures to be predicted without the need for MS analysis, the dereplication of known compounds and identification of new compounds based solely on parameters derived by DOSY NMR.

We report accurate DOSY NMR based molecular weight and diffusion coefficient prediction tools. These tools can be used to dereplicate known natural products from databases using structurally rich NMR data as a surrogate for mass spectrometric data.     

Silicon-29 NMR studies of aqueous silicate solutions: Part II. Isotopic enrichment

Silicon-29 NMR spectra have been recorded for aqueous solutions of sodium silicate in the alkaline pH range using silica enriched in the ²⁹Si isotope. Work at low concentration shows that essentially only monomeric orthosilicate ions are present at ca. 0.01 M in silica. At higher concentrations, peaks which lack fine structure indicate species whose silicon nuclei are all equivalent. The experiments have assisted in the difficult process of assigning the spectra of silicate solutions.     

Local covariance order diffusion-ordered spectroscopy: a powerful tool for mixture analysis

Diffusion-ordered spectroscopy (DOSY) is an important tool in NMR mixture analysis that has found use in most areas of chemistry, including organic synthesis, drug discovery, and supramolecular chemistry. Typically the aim is to disentangle the overlaid, and often overlapped, NMR spectra of individual mixture components and/or to obtain size and interaction information from their respective diffusion coefficients. The most common processing method, high-resolution DOSY, breaks down where component spectra overlap; here multivariate methods can be very effective, but only for small numbers (2-5) of components. In this study, we present a hybrid method, local covariance order DOSY (LOCODOSY), that breaks a spectral data set into suitable windows and analyzes each individually before combining the results. This approach uses a multivariate algorithm (e.g., SCORE or DECRA) to resolve only a small number of components in any given window. Because a small spectral region should contain signals from only a few components, even when the spectrum as a whole contains many more, the total number of resolvable chemical components rises dramatically. It is demonstrated here that complete resolution of component spectra can be achieved for mixtures that are much more complex than could previously be analyzed with DOSY. Thus, LOCODOSY is a powerful, flexible tool for processing NMR diffusion data of complex mixtures.     

29Si chemical shift anisotropies in calcium silicates from high-field 29Si MAS NMR spectroscopy

29Si chemical shift anisotropy (CSA) data have been determined from (29)Si MAS NMR spectra recorded at 14.1 T for a number of synthetic calcium silicates and calcium silicate hydrates. These are beta- and gamma-Ca(2)SiO(4), Ca(3)SiO(4)Cl(2), alpha-dicalcium silicate hydrate (alpha-Ca(2)(SiO(3)OH)OH), rankinite (Ca(3)Si(2)O(7)), cuspidine (Ca(4)Si(2)O(7)F(2)), wollastonite (beta-Ca(3)Si(3)O(9)), pseudowollastonite (alpha-Ca(3)Si(3)O(9)), scawtite (Ca(7)(Si(6)O(18))CO(3).2H(2)O), hillebrandite (Ca(2)SiO(3)(OH)(2)), and xonotlite (Ca(6)Si(6)O(17)(OH)(2)). The (29)Si MAS NMR spectra of rankinite and wollastonite clearly resolve manifolds of spinning sidebands from two and three Si sites, respectively, allowing the CSA parameters to be obtained with high precision for each site. For the (29)Si Q(1) sites in rankinite and cuspidine, the CSA asymmetry parameters (eta(sigma) approximately 0.6) contrast the general expectation that sorosilicates should possess small eta(sigma) values as a result of the nearly axially symmetric environments of the SiO(4) tetrahedra. The (29)Si CSA parameters provide an improved insight into the electronic and geometric environments for the SiO(4) tetrahedra as compared to the values solely for the isotropic chemical shift. It is shown that the shift anisotropy (delta(sigma)) and the CSA asymmetry parameter (eta(sigma)) allow a clear distinction of the different types of condensation of SiO(4) tetrahedra in calcium silicates. This relationship may in general be valid for neso-, soro-, and inosilicates. The CSA data determined in this work may form a valuable basis for (29)Si MAS NMR studies of the structures for tobermorites and calcium silicate hydrate phases resulting from hydration of Portland cements.     

Matrix isolation vibrational spectroscopy as a tool for studying conformational isomerism

The use of matrix isolation vibrational spectroscopy to study conformational isomerism is described. Methods of distinguishing conformational splitting from matrix splitting are discussed. Examples are given of molecules for which the conformational equilibrium existing in the gas phase prior to deposition is trapped out in the matrix, of molecules which exhibit reversible interconversion of conformers at matrix temperatures, and of a molecule for which the conformational distribution trapped out in the matrix is strongly dependent on matrix polarity. Results obtained for molecules which exhibit infrared-induced isomerisation in matrices are discussed.     

ERETIC implemented in diffusion-ordered NMR as a diffusion reference

The ERETIC (Electronic Reference To access In vivo Concentrations) technique generates an electronic signal in the NMR spectrometer which is detected simultaneously to the sample FID during the acquisition. The implementation of the ERETIC sequence in any 2D DOSY experimental scheme enables one to generate directly into the raw 2D DOSY spectrum a reference signal with an attenuation simulated to describe a well-defined diffusion behavior. This simulated intensity attenuation can be used to evaluate the output generated by any DOSY data treatment algorithm, in a single as well as multichannel approach and provide insight into their precision, accuracy, scope and limitations. The ERETIC sequence implemented in the standard bipolar pulsed field gradient longitudinal eddy current delay (LED) sequence is illustrated on various algorithms presented previously in the literature for the analysis of the generated ERETIC-DOSY spectra of simulated model systems representing discrete and continuous diffusion profiles in mono- and bi-Gaussian diffusion regimes.     

79Br NMR spectroscopy as a practical tool for kinetic analysis

⁷⁹Br NMR spectroscopy has been used to monitor a series of reactions in which the bromide ion is produced, including the Menschutkin reaction of pyridine with a range of substituted benzyl bromides and a Heck coupling process. In cases where the process could also be monitored using ¹H NMR spectroscopy, the kinetic analyses using heteronuclear magnetic resonance spectroscopy were shown to be completely consistent. Both the utility of the process in following reactions which may be difficult to analyse using other techniques and the practical limitations associated with solvent choice are discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

ERETIC implemented in diffusion-ordered NMR as a diffusion reference: a clarification

Additional information as to the objectives of our previous paper entitled 'ERETIC implemented in diffusion-ordered NMR as a diffusion reference' (Magn. Reson. Chem. 2008, 46, S63) is provided. The need for an optimal instrumental stability for the method proposed to be reliably applicable in view of these objectives is emphasized and illustrated.     

Mass spectrometry as a tool for studying tautomerism

The review contains new data on tautomerism of organic compounds belonging to different classes, which were obtained by mass spectrometry and confirmed by quantum-chemical calculations. Published in Russian in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 12, pp. 1751–1763. The text was submitted by the authors in English.     

Detection of environmental polyorganosiloxanes (silicones) by silicon-29 NMR spectroscopy

The utility of ²⁹Si NMR spectroscopy has been demonstrated on sediment-like materials in the quantitative and qualitative determination of polyorganosiloxanes (silicones) in selected environmental samples. This technique is highly selective for polydimethylsiloxanes (PDMS) and is non-destructive to the sample. Also, specific identification of polyorganosiloxanes in sediment is possible, in contrast to previous methods which provided only quantitative information while consuming the sample. The detection limit for a 9 h experiment is approximately 45 ppm.     

Silicon-29 NMR studies of polymethylhydrosiloxanes: Spin-lattice relaxation time (T1) measurements

²⁹Si NMR spectra of polymethylhydrosiloxanes, Me₃SiO[MeHSiO]_nSiMe₃ from n = 3 to 8 and 35, have been determined. Both chemical shifts and spin-lattice relaxation times (T₁) have been measured. The stereochemistry at the adjacent chiral MeHSiO unit influences the nearest neighbor ²⁹Si chemical shift. The effect of chain length and position of MeHSiO units on T₁ values for Me₃SiO[MeHSiO]_nSiMe₃ systems are discussed.     

Donor-donor energy migration (DDEM) as a tool for studying aggregation in lipid phases

A BODIPY-labelled sulfatide (N-(BODIPY-FL-pentanoyl)-galactosylcerebroside-sulfate, hereafter abbreviated as BD-Sulfatide) was solubilised at different concentrations in lipid vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Time-correlated single photon counting experiments show that the fluorescence relaxation is mono-exponential (with a lifetime of 6.5 ns) at molar ratios of BD-Sulfatide: DOPC that are less than 1:100. The fluorescence steady-state anisotropy decreases monotonously at molar ratios smaller than 1:1000, which is compatible with donor-donor energy migration (DDEM) among the BODIPY groups. A model that assumes DDEM across the lipid bilayers, as well as in their planes, was used to analyse the time-resolved fluorescence anisotropy. Only two parameters appear in the model namely: the bilayer thickness (d) and the average number density (C2) distribution of BD-Sulfatide in the lipid bilayers. The extracted d-values vary between 35 and 40 A, which is about the reported thickness of a bilayer of DOPC (38 A). Hence, the BODIPY groups are preferentially located in the water-lipid interface. At low concentration the experimental C2-values and those independently calculated are in good agreement, while the experimental values gradually become lower with increasing BD-Sulfatide concentration. These results are compatible with an aggregation of the sulfatides and self-quenching of BODIPY, which is clearly established at higher concentrations of the BD-Sulfatide.     

Calculation of the (29)si NMR chemical shifts of aqueous silicate species

A DFT methodology for calculating (29)Si NMR chemical shifts of silicate species typically present prior to nucleation in zeolite synthesis solutions, incorporating solvent effects through an implicit representation is presented. We demonstrate how our methodology can reproduce the experimentally observed spectra and, by comparison to well characterized peaks in two different experimental studies, demonstrate the transferability and robustness of the methodology. We discuss certain cases in which caution must be exercised when implicit solvent representations are used for calculating silicate cluster geometries: those cases in which intramolecular hydrogen bonding can play a significant role in the geometry. A number of reassignments of previous tentative experimental assignments are proposed, and we also make assignments for the challenging substituted four-ring species. We present all of our computed chemical shift for previously observed species together with a number of other viable silicate clusters to serve as a reference point for future experimental studies.     

NMR imaging as a new tool for rheology

Examples of NMR imaging used to study the evolution of microstructure and flow velocities in sheared, highly filled suspensions are described. Fast NMR imaging methods were used to freeze the motion in a falling-ball experiment, allowing us to monitor the local concentrations of suspended particles and ball position during the course of the experiment The migration of particles induced by shear and concentration gradients was followed in a Couette cell. Flow imaging methods were developed and applied to a Newtonian fluid and a non-Newtonian suspension flowing in an axisymmetric pipe contraction.     

DOSY NMR as a tool for predicting optimal conditions for hydrogel formation: The case of a hyperbranched polyglycidol cross‐linked with boronic acids

This article demonstrates the utility of DOSY NMR for the determination of the optimal conditions for the efficient covalent, reversible cross‐linking of macromolecules in water for hydrogel formation. The studied model system was hyperbranched polyglycidol (HbPGL) containing numerous diol groups in peripheral regions and two types of boronic acids, that is, B(OH)₄^? and benzene‐1,4‐boronic diacid, as cross‐linking agents. Diffusion coefficient changes of a polymer in solution, under the influence of various concentrations of cross‐linking agent and pH, which influences the equilibrium of the reaction between boronic acids and diols, were recorded. These data are consistent with the rheological properties, namely the G′_max(ω) of hydrogels prepared under analogous conditions, from more concentrated solutions of HbPGL. This approach appears to be promising as it facilitates avoiding the loss of a large amount of polymer that is necessary for the elaboration of appropriate conditions for network formation in aqueous media. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2171–2178