Gerard J. Martin

Consistency of NMR and mass spectrometry determinations of natural-abundance site-specific carbon isotope ratios. The case of glycerol.

Quantitative determinations of natural-abundance carbon isotope ratios by nuclear magnetic resonance (SNIF-NMR) have been optimized by appropriate selection of the experimental conditions and by signal analysis based on a dedicated algorithm. To check the consistency of the isotopic values obtained by NMR and mass spectrometry (IRMS) the same glycerol samples have been investigated by both techniques. To have access to site-specific isotope ratios by IRMS, the products have been degraded and transformed into two derivatives, one of which contains carbons 1 and 3 and the other carbon 2 of glycerol. The sensitivity of the isotopic parameters determined by IRMS to fractionation effects possibly occurring in the course of the chemical transformations has been investigated, and the repeatability and reproducibility of both analytical chains have been estimated. The good agreement observed between the two series of isotopic results supports the reliability of the two different approaches. SNIF-NMR is therefore a very attractive tool for routine determination, in a single nondestructive experiment, of the carbon isotope distribution in glycerol, and the method can be applied to other compounds. Using this method, the isotopic distributions have been compared for glycerol samples, obtained from plant or animal oils, extracted from fermented media, or prepared by chemical synthesis. Typical behaviors are characterized.

Nuclear magnetic resonance investigations of carbonium ion intermediates. Part II. Exchange reactions in chloro-iminium salts (Vilsmeier–Haack reagents)

Different kinds of coalescence phenomena have been observed for solutions of chloro-iminium salts. The rate of the exchange mechanism is concentration dependent. An acid–base equilibrium between the Vilsmeier complex (Me2NCHCl)+B– and the dimethylformamide is evident. Thermodynamic and kinetic parameters have been calculated for different solvents and different anions B– by total line shape analysis of continuous wave and Fourier transform spectra.

Nuclear magnetic resonance investigations of carbonium ion intermediates. Part I. Kinetics and mechanism of formation of the Vilsmeier–Haack reagent

The mechanism of formation and the structure of the Vilsmeier reagent (dimethylformamide and POCl3, SOCl2, or COCl2) has been studied by use of kinetic and structural data obtained from 1H and 31P n.m.r. spectroscopy. The rate constants and activation energies are discussed in terms of structural changes and solvent effects.

Nuclear magnetic resonance investigations of carbonium ion intermediates. Part III. A chlorine-35 quadrupole resonance study of several (R-chloromethylene)dimethylammonium salts (Vilsmeier–Haack and Viehe reagents)

The 35Cl n.q.r. frequencies observed in a series of solid chloromethylenedimethylammonium salts are in agreement with the general structure (i). The variations in the C–35Cl resonance frequency indicate a large polarisability [graphic omitted] of the [NC]+ group, both with respect to the σ and π electron systems. The charge on the Cl atom increases in the order R = Cl < Ph < Pri < H.

Solvent effects on the spin-spin coupling constants between geminal allenic protons

Abstract The coupling constant between geminal allenic protons was studied through JHCD. A significant solvent dependence was observed. With allenic ketones, RCOCHCCH2, | JHCH increased with the dielectric constant of solvent. This coupling constant is negative and the experimental changes satisfy the correlations of solvement effects on direction and sign of J.

Natural selective 2H labelling applied to the study of chemical mechanisms; labelling without enrichment

It is shown that the internal distribution deuterium may considerably differ from the statistical distribution and that this property is exploitable, by quantitative 2H n.m.r. measurements, for investigating reaction mechanisms using isotopic labeling without the need for selective enrichment.

Nuclear magnetic resonance investigation of iminium ion intermediates. Part VI. A hydrogen-1 and carbon-13 structural and dynamic study of various substituted iminium salts

1 H an 13C chemical shifts and 1J(13C–H) of various substituted iminium salts and parent compounds are presented and correlated with the substituent parameters σI and σR and the energies of electronic transitions λπ→π*. A 13C dynamic study shows that the rotation about the C–N bond is generally slow with respect to the n.m.r. time-scale and T1(13C) measurements are applied to a qualitative description of the molecular motion of iminium salts. The results are discussed in terms of ion-pair constitution.

Authentication of natural flavours using SNIF-NMR® new developments on mustard oil and safron

Abstract In several industrialised countries, worldwide, “natural” food enjoys a growing success. Following this trend Flavour manufactures strive to provide natural flavours and essential oils to their customers in the food and beverages industry. Unfortunately, it has frequently occurred that chemically produced (nature-identical) molecules have been used (often unknowingly) without declaration as cheaper substitutes for natural flavours. A wide range of methods have therefore been developed to check the natural origin of many molecules constituting flavours. Among these techniques, Isotopic Methods (13C, 2H, 14C) occupy a prime position. Unfortunately, it has been reported that the most commonly used methods (14C counting and 13C SIRA analyses) can be easily countered by adulterators by addition of isotopically enriched compounds. The SNIF-NMR® method which provides isotopic ratios for each molecular position within a molecule has more recently been proposed as an uncounterfeitable fingerprint of natural molecules. Initially applied to ethanol, anethole and vanillin the SNIF-NMR® method has more recently been applied to a wide range of other natural molecules. This presentation will present sensitivity improvements of the SNIF-NMR® method for important molecules as well as new developments on safron and mustard oil.

A comparative multinuclear 1H, 13C, and 15N magnetic resonance study of organic thiocyanates and isothiocyanates

1 H, 13C, and 15N n.m.r. spectra of thiocyanato (RSCN) and isothiocyanato (RNCS) compounds, especially for the vinylic series R1C(SCN)=CHR2 and R1C(NCS)=CHR2, are reported. 15N Chemical shifts of thiocyanates (δca.–100 p.p.m.) and of isothiocyanates (δca.–275 p.p.m.) are very different, the value for the thiocyanate ion (SCN)– being intermediate (δ–165 p.p.m.). Comparison of the spectra of vinyl thiocyanates and isothiocyanates with those of the corresponding saturated compounds suggests that the SCN group exerts both attractive σ and π effects on the CC bond mainly through the intervention of the d orbitals of the S atoms. The NCS group appears to be slightly electron donating towards the double bond. Solvent effects on the 15N chemical shift of BunSCN and BunNCS are discussed in terms of Tafts linear solvation energy relationships.

Nuclear magnetic resonance studies of iminium salts. Part 11. Anionic participation of iminium salts in Phosgenation reactions

Iminium salts act as catalysts in the reaction of phosgene and phenols or thiols to yield chloroformates, which are important synthetic intermediates. This effect is explained in terms of the nucleophilicity of the chloride anion of the salts on the basis of an n.m.r. identification of various intermediates. The mechanism of nucleophilic assistance of iminium salts towards the substrate can be understood as an increase of the nucleophilicity of phenol or thiol by proton abstraction from the OH or SH group by the chloride anion.