Isotope ratio monitoring gas chromatography/Mass spectrometry of D/H by high temperature conversion isotope ratio mass spectrometry

Of all the elements, hydrogen has the largest naturally occurring variations in the ratio of its stable isotopes (D/H). It is for this reason that there has been a strong desire to add hydrogen to the list of elements amenable to isotope ratio monitoring gas chromatography/mass spectrometry (irm-GC/MS). In irm-GC/MS the sample is entrained in helium as the carrier gas, which is also ionized and separated in the isotope ratio mass spectrometer (IRMS). Because of the low abundance of deuterium in nature, precise and accurate on-line monitoring of D/H ratios with an IRMS requires that low energy helium ions be kept out of the m/z 3 collector, which requires the use of an energy filter. A clean mass 3 (HD(+.)) signal which is independent of a large helium load in the electron impact ion source is essential in order to reach the sensitivity required for D/H analysis of capillary GC peaks. A new IRMS system, the DELTA(plus)XL(trade mark), has been designed for high precision, high accuracy measurements of transient signals of hydrogen gas. It incorporates a retardation lens integrated into the m/z 3 Faraday cup collector. Following GC separation, the hydrogen bound in organic compounds must be quantitatively converted into H(2) gas prior to analysis in the IRMS. Quantitative conversion is achieved by high temperature conversion (TC) at temperatures >1400 degrees C. Measurements of D/H ratios of individual organic compounds in complicated natural mixtures can now be made to a precision of 2 per thousand (delta notation) or, better, with typical sample amounts of approximately 200 ng per compound. Initial applications have focused on compounds of interest to petroleum research (biomarkers and natural gas components), food and flavor control (vanillin and ethanol), and metabolic studies (fatty acids and steroids). Copyright 1999 John Wiley & Sons, Ltd.     

1,2-Halogen migration in haloallenyl ketones: regiodivergent synthesis of halofurans

Selective 1,2-iodine, bromine, and chlorine migration in haloallenyl ketones in the presence of Au catalyst has been demonstrated. It was found that, depending on the nature of the Au catalyst used, either selective bromine migration or hydrogen shift occurs, leading to the formation of 3- or 2-bromofurans, respectively. Halirenium intermediate was proposed for the unusual 1,2-halogen migration. This cascade transformation allows for mild and efficient synthesis of various types of 3-halofurans.     

Extended-ion approximation and pressure shifts inF-band energies

Pressure variation of maximumF-band absorption energies in the halides of lithium, sodium and potassium has been investigated employing the extended-ion approximation for the calculation of theF-electron energy eigenvalues and using values of local compressibility in the neighbourhood of theF-centres which include the effect of vacancy and pressure. The results obtained agree with the experimental results.     

Palladium-catalyzed arylation and heteroarylation of indolizines

A highly effective protocol for palladium-catalyzed selective arylation and heteroarylation of indolizines at C-3 has been developed. Mechanistic studies unambiguously support an electrophilic substitution pathway for this transformation.     

Metal-catalyzed 1,2-shift of diverse migrating groups in allenyl systems as a new paradigm toward densely functionalized heterocycles

A general, mild, and efficient 1,2-migration/cycloisomerization methodology toward multisubstituted 3-thio-, seleno-, halo-, aryl-, and alkyl-furans and pyrroles, as well as fused heterocycles, valuable building blocks for synthetic chemistry, has been developed. Moreover, regiodivergent conditions have been identified for C-4 bromo- and thio-substituted allenones and alkynones for the assembly of regioisomeric 2-hetero substituted furans selectively. It was demonstrated that, depending on reaction conditions, ambident substrates can be selectively transformed into furan products, as well as undergo selective 6-exo-dig or Nazarov cyclizations. Our mechanistic investigations have revealed that the transformation proceeds via allenylcarbonyl or allenylimine intermediates followed by 1,2-group migration to the allenyl sp carbon during cycloisomerization. It was found that 1,2-migration of chalcogens and halogens predominantly proceeds via formation of irenium intermediates. Analogous intermediate can also be proposed for 1,2-aryl shift. Furthermore, it was shown that the cycloisomerization cascade can be catalyzed by Br?nsted acids, albeit less efficiently, and commonly observed reactivity of Lewis acid catalysts cannot be attributed to the eventual formation of proton. Undoubtedly, thermally induced or Lewis acid-catalyzed transformations proceed via intramolecular Michael addition or activation of the enone moiety pathways, whereas certain carbophilic metals trigger carbenoid/oxonium type pathway. However, a facile cycloisomerization in the presence of cationic complexes, as well as observed migratory aptitude in the cycloisomerization of unsymmetrically disubstituted aryl- and alkylallenes, strongly supports electrophilic nature for this transformation. Full mechanistic details, as well as the scope of this transformation, are discussed.     

Diagrammatic structure of the general coupled cluster equations

The general formula for the number of diagrammatic terms occurring in the T_n equation within a particular coupled cluster model is derived. Both the antisymmetrized and Goldstone diagrams are considered. In addition to the full coupled cluster equation approximate approaches are discussed, and for each the general formula for the number of terms is given. Analogous expressions are presented for the number of diagrammatic terms contributing to the elements of the transformed Hamiltonian [Hbar] = e^?T He^T .