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.     

Real versus artifactual symmetry-breaking effects in Hartree-Fock, density-functional, and coupled-cluster methods

We have examined the relative abilities of Hartree-Fock, density-functional theory (DFT), and coupled-cluster theory in describing second-order (pseudo) Jahn-Teller (SOJT) effects, perhaps the most commonly encountered form of symmetry breaking in polyatomic molecules. As test cases, we have considered two prototypical systems: the 2Sigmau+ states of D( infinity h) BNB and C3+ for which interaction with a low-lying 2Sigmag+ excited state leads to symmetry breaking of the nuclear framework. We find that the Hartree-Fock and B3LYP methods correctly reproduce the pole structure of quadratic force constants expected from exact SOJT theory, but that both methods appear to underestimate the strength of the coupling between the electronic states. Although the Tamm-Dancoff (CIS) approximation gives excitation energies with no relationship to the SOJT interaction, the random-phase-approximation (RPA) approach to Hartree-Fock and time-dependent DFT excitation energies predicts state crossings coinciding nearly perfectly with the positions of the force constant poles. On the other hand, the RPA excited-state energies exhibit unphysical curvature near their crossings with the ground (reference) state, a problem arising directly from the mathematical structure of the RPA equations. Coupled-cluster methods appear to accurately predict the strength of the SOJT interactions between the 2Sigmau+ and 2Sigmag+ states, assuming that the inclusion of full triple excitations provides a suitable approximation to the exact wave function, and are the only methods examined here which predict symmetry breaking in BNB. However, coupled-cluster methods are plagued by artifactual force constant poles arising from the response of the underlying reference molecular orbitals to the geometric perturbation. Furthermore, the structure of the "true" SOJT force constant poles predicted by coupled-cluster methods, although correctly positioned, has the wrong structure.     

A comparative study of the rearrangement of some 6- and 7-halo-substituted 3-amino-3,4-dihydro-l-hydroxycarbostyrils in concentrated hydrohalic acids

The 6-bromo ( 16 ), 6-fluoro ( 17 ), 7-bromo ( 14 ), and 7-fluoro ( 15 ) substituted 3-amino-3,4-dihydro-l-hydroxycarbostyrils were treated with concentrated hydrochloric and hydrobromic acids under reflux conditions. The 7-halogenated N-hydroxycarbostyrils ( 14,15 ) gave the normal rearrangement products, the 6,7-dihalolactams ( 18–21 ). The 6-halogenated compounds ( 16,17 ) yielded the corresponding 6,8-dihalolactams ( 22–24 ) under the same experimental conditions, with the exception of the hydrobromic acid reaction of the 6-fluoro derivative 17 which yielded a mixture of products. Based on the comparison of the nmr spectrum of the product mixture with those of two authentic compounds, the mixture was identified as consisting of the normal rearrangement product, the 8-bromo-6-fluorolactam ( 27 ) and the straightforward reduction product, the 6-fluorolactam ( 26 ) in a ratio of about 2:1. The latter compounds were prepared by an independent method of synthesis in which 2-amino-5-fluorophenylalanine ( 25 ) was acidified to yield the corresponding lactam 26 , followed by bromination to afford the 8-bromo-6-fluorolactam 27. A mechanism is proposed to interpret the experimental results of nucleophilic substitution with rearrangement and reduction which occur with the 6-fluoro compound 17 when exposed to bromide ions in strongly acidic solution.