Conformational analysis of rigid enol-ethers by proton nuclear magnetic resonance

Z,E isomers of rigid enol-ethers were studied by proton nuclear magnetic resonance. As with oximes, it was found that the chemical shift difference (Δδ=δ_z?δ_E) for the protons α to the function in question depends on the dihedral angle between the C_α? H and C?C bonds. This phenomenon can be explained by an electric field effect and not by a magnetic anisotropy effect. The present study has allowed the derivation of values of the volume magnetic susceptibility and the product of b×the dipole moment for the C? O bond.     

Quantitative analysis by nuclear magnetic resonance using precision coaxial tubing

Quantitative analysis by n.m.r. using precision coaxial tubing is described. A solution of the intensity standard is placed in the central capillary and the sample of interest in the surrounding annulus. By this method the contamination of the sample by the standard compound can be avoided and an accurate determination can be carried out by using solutions of correct concentrations of the intensity standard. Applications to several types of quantitative problems are given. In order to get a higher accuracy and precision the experimental conditions were also studied.     

Computer-aided carbon-13 nuclear magnetic resonance spectroscopy for identification of terpenoid mixtures

The procedure for identifying triterpenes in mixtures is based on the simulation of ¹³C-NMR spectra for probable mixtures and comparison of these with a specialized spectral data bank. The system was designed to facilitate the analysis of complex mixtures of terpenoid compounds. A special matching procedure was developed and its efficiency is discussed. The method is demonstrated for a mixture of five triterpenes isolated from Vernonia cognata.     

Spherical tensor analysis of nuclear magnetic resonance signals

In a nuclear magnetic-resonance (NMR) experiment, the spin density operator may be regarded as a superposition of irreducible spherical tensor operators. Each of these spin operators evolves during the NMR experiment and may give rise to an NMR signal at a later time. The NMR signal at the end of a pulse sequence may, therefore, be regarded as a superposition of spherical components, each derived from a different spherical tensor operator. We describe an experimental method, called spherical tensor analysis (STA), which allows the complete resolution of the NMR signal into its individual spherical components. The method is demonstrated on a powder of a (13)C-labeled amino acid, exposed to a pulse sequence generating a double-quantum effective Hamiltonian. The propagation of spin order through the space of spherical tensor operators is revealed by the STA procedure, both in static and rotating solids. Possible applications of STA to the NMR of liquids, liquid crystals, and solids are discussed.     

Probing beer aging chemistry by nuclear magnetic resonance and multivariate analysis

This paper describes the use of nuclear magnetic resonance (NMR) spectroscopy, in tandem with multivariate analysis (MVA), for monitoring the chemical changes occurring in a lager beer exposed to forced aging (at 45 °C for up to 18 days). To evaluate the resulting compositional variations, both principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) were applied to the NMR spectra of beer recorded as a function of aging and a clear aging trend was observed. Inspection of PLS-DA loadings and peak integration enabled the changing compounds to be identified, revealing the importance of well known markers such as 5-hydroxymethylfurfural (5-HMF) as well as a range of other relevant compounds: amino acids, higher alcohols, organic acids, dextrins and some still unassigned spin systems. In addition, the multivariate analysis method of 2D correlation analysis was applied to the NMR data enabling the relevant compound variations to be confirmed and inter-compound correlations to be assessed, some reflecting common metabolic/chemical pathways and, therefore, offering improved insight into the chemical aspects of beer aging.     

The determination of aromatic substitution patterns by nuclear magnetic resonance

A method is described which permits the analysis of many substituted benzene compounds. The method is useful for mono-through tetra- substituted aromatics. In ideal cases, the number and disposition of the substituents and the chemical shifts of the residual protons can be accurately determined. The method assumes approximate first order couplings and is useful for roughly 50% of all aromatics. Frequently observed anomalous spectra are also discussed.     

Anionic-nonionic surfactant interaction by nuclear magnetic resonance

Summary The interaction between anionic (sodium dodecyl benzene sulfonate) surfactant and nonionic (Tri and Tetra propylene glycol monomethyl ether) surfactant was studied using nuclear magnetic resonance measurement. It was observed that the addition of sodium dodecyl benzene sulfonate to the solution of nonionic surfactant (Tri and Tetra propylene glycol mono methyl ether) caused an upfield shift of the central protons of the nonionic surfactants. The aromatic protons of sodium-dodecyl benzene sulfonate undergo a very small, almost negligible, downfield shift. The changes in the chemical shift values and the integration values of the polypropylene protons and benzene protons was interpreted in terms of mixed micelle formation with the simultaneous presence of highly fluid mixed micelles of varying compositions.With 2 figures and 2 tables     

Prediction of physical properties of hydrocarbon mixtures by partial-least-squares calibration of carbon-13 nuclear magnetic resonance data

By using the partial-least-squares (PLS) method, bulk properties of 12-component synthetic mixtures containing n-alkanes, iso-alkanes, cyclo-alkanes and aromatics are calibrated against intensities and chemical shifts of ¹³C-NMR spectra. The standard error of prediction (SEP) for the determinations of density, refractive index, mean molecular weight and carbon-type distribution was found to be less than 3.2% of the observed range. The SEP for excess densities is significantly larger, especially for values based on chemical shift data. The chemical shift variation supplies unique chemical information on solute/solvent interactions.     

Quantitative analysis in pharmacy and pharmaceutical chemistry by nuclear magnetic resonance spectroscopy

The applications of n.m.r. to the quantitative analysis of pharmaceutical formulations and products of interest to the pharmacist and pharmaceutical analyst are reviewed. Special attention is paid to the accuracy of the method, the coefficients of variation being quoted (or calculated from data in the original paper) where possible. An elementary knowledge of n.m.r. is assumed.     

Characterization of cellulose acetates by nuclear magnetic resonance

An NMR method for determining the distribution of acetyl groups in cellulose acetates was developed. Treatment of cellulose acetates with acetyl-d₃ chloride gave products having simple spectra which could be analyzed quantitatively to give the distribution of acetyl groups in the original sample. The method was applied to studying (1) the hydrolysis of cellulose triacetate with ammonia, (2) the acetylation of cellulose acetate with acetyl chloride, and (3) the acetylation of cellulose acetate with acetic anhydride.     

Self-diffusion of water-ethanol mixtures in polyacrylic acid-polysulfone composite membranes obtained by pulsed-field gradient nuclear magnetic resonance spectroscopy

The self-diffusion of water, ethanol and water-ethanol mixtures in polyacrylic acid (PAA) and PAA-polysulfone (PSF) composite membranes was measured with pulsed-field gradient (PFG)-nuclear magnetic resonance (NMR) spectroscopy. The partial solubilities and self-diffusion coefficients were obtained. An attempt was made to explain the transport properties of water and ethanol through the PAA layer and PAA-PSF composite membranes. It was concluded that there are two types of channels for diffusate transfer in PAA: an ionogenic hydrophilic channel which is selective for water and a hydrophobic channel which contains a predominance of ethanol molecules. The existence of aluminum nitrate in PAA influences the morphology of the latter channel. There is a good agreement between of the separation factors estimated from PFG-NMR data and those obtained by pervaporation testing.     

The study of domain structure in polyurethanes by nuclear magnetic resonance

Polyurethanes are block copolymers composed of hard and soft segments which often undergo microphase separation with the resultant formation of hard and soft domains. The nuclear-magnetic-free induction decay of such systems is shown to consist of a fast Gaussian component corresponding to glassy domains and a slow exponential component corresponding to rubbery domains. Thus, by being sensitive to the microscopic mobility of a material, nuclear magnetic resonance (NMR) has the potential to determine when a polyurethane has separated into domains and the relative amounts of material in each domain. After annealing the free-induction decay of a linear polyurethane showed a distribution of relaxation times indicating that mixing of the domains had taken place and a continuum of compositions exist in the material. The free-induction decay of a cross-linked polyurethane was unaffected by annealing, confirming the results of an earlier study.     

Structural analysis of unstable norbixin isomers guided by pure shift nuclear magnetic resonance

We report the analysis of complex samples obtained during the microwave irradiation/heating of norbixin, which has been identified as a potential therapeutic target for age-related macular degeneration (AMD). In this context, identifying the different isomers that are obtained during its degradation is of primary importance. However, this characterization is challenging because, on the one hand, some of these isomers are unstable, and on the other hand, the ¹H spectra of these isomeric mixtures are poorly resolved. We could successfully apply 1D pure shift experiments to obtain ultrahigh-resolution ¹H nuclear magnetic resonance (NMR) spectra of the norbixin isomer samples and exploit their information content to analyze complementary 2D NMR data and describe accurately their isomeric composition.     

Sequence analysis of ethylene sulphide-propylene sulphide copolymers by carbon-13 nuclear magnetic resonance

Ethylene sulphide-propylene sulphide copolymers, prepared with anionic type catalysts, have been investigated by ¹³C NMR. The peaks have been assigned both in terms of diad and triad monomer sequences and in terms of tacticity. It has been shown that the propylene sulphide monomer units were directionally oriented and the tacticity was approximately random. We have also determined the effect of methyl substitution in various positions on the chemical shift of the main chain carbon atoms.