A theoretical study of the barbier reaction

Semiempirical MO calculations are shown to provide a more detailed picture of the radical ion mechanism of the Barbier reaction in the general case. Calculated electron affinities give a clue for its extension to other cases.     

Topological localized molecular orbitals

Intrinsic and external π-orbital localization procedures which rely only on molecular topology are proposed and discussed. Localized molecular orbitals obtained by application of these procedures are referred to as ‘topological localized molecular orbitals”.     

Molecular potential energy surfaces by interpolation: strategies for faster convergence

A method for interpolating molecular potential energy surfaces introduced [Ischtwan and Collins, J. Chem. Phys. 100, 8080 (1994)] and developed as an iterative scheme has been improved by different criteria for the selection of the data points. Refinements in the selection procedure are based on the variance of the interpolation and the direct exploration of the interpolation error, and produce more accurate surfaces than the previously established scheme for the same number of data points.     

Ion-trap tandem mass spectrometry for the determination of heterocyclic amines in food

Heterocyclic amines (HAs) are mutagenic compounds to which humans are regularly exposed through diet. Due to the high complexity of the sample matrix and the low level of concentration of HAs, sensitive and selective analytical methodologies are required. Here we describe a methodology based on liquid chromatography-atmospheric pressure chemical ionisation tandem mass spectrometry using an ion-trap to analyse HAs. The collision-induced dissociation parameters for tandem ion-trap spectrometric analysis of these mutagenic compounds were optimised, and the full scan MS-MS spectra were used for unequivocal identification of the analytes. For aminoimidazoazaarenes, the most abundant ions were derived from the loss of a methyl group and the breaking of the aminoimidazole moiety, while for carbolines the major product ions arose from the loss of ammonia and HCN. Moreover, the performance of the LC-atmospheric pressure chemical ionisation MS-MS method was evaluated. The good precision (RSD lower than 11%) and the low detection limits achieved (10-60 pg injected) allow the determination of HAs at low part-per-billion level (0.4-5.0 ng g(-1)) in a lyophilised meat extract.     

Determination of naphthalene-derived compounds in apples by ultra-high performance liquid chromatography-tandem mass spectrometry

Naphthylacetic acid, naphthyloxy acetic acid and naphthylacetamide belong to a group of synthetic substances known as “auxin-like” compounds which are used as growth regulators in vegetables and fruits due to their structure similarities with the indoleacetic acid, the most important plant auxin. This paper reports a selective, sensitive and fast ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC–MS/MS) method for the determination of naphthylacetamide (NAD) and the isomers (α and β) of naphthylacetic acid (NAA) and naphthyloxy acetic (NOA) acid in apple samples. A baseline separation between the respective isomers was achieved using an RP-Amide column with gradient elution. The UHPLC-MS/MS method developed, using electrospray and selected reaction monitoring (SRM) acquisition mode led to a reliable determination of these family of compounds in apple samples at low quantitation levels, down to 1.0 μg kg⁻¹ and 0.25 μg kg⁻¹ respectively. For confirmation of NAA accurate mass measurement is proposed giving at these conditions quantitation limits of 10 μg kg⁻¹ for this compound. The UHPLC-MS/MS method developed was used for the analysis of apple samples harvested in three different apple fields from Lleida (Spain) during the blooming period. NAD and NAA were found in samples collected during 4–5 weeks after application at concentrations between the quantification limits and 43 μg kg⁻¹ and 24 μg kg⁻¹, respectively.     

Towards J/mol Accuracy for the Cohesive Energy of Solid Argon

The cohesive energies of argon in its cubic and hexagonal closed packed structures are computed with an unprecedented accuracy of about 5 J mol^?1 (corresponding to 0.05 % of the total cohesive energy). The same relative accuracy with respect to experimental data is also found for the face‐centered cubic lattice constant deviating by ca. 0.003 Å. This level of accuracy was enabled by using high‐level theoretical, wave‐function‐based methods within a many‐body decomposition of the interaction energy. Static contributions of two‐, three‐, and four‐body fragments of the crystal are all individually converged to sub‐J mol^?1 accuracy and complemented by harmonic and anharmonic vibrational corrections. Computational chemistry is thus achieving or even surpassing experimental accuracy for the solid‐state rare gases.     

Cloud point extraction of mercury with PONPE 7.5 prior to its determination in biological samples by ETAAS

Cloud point extraction (CPE) has been used for the pre-concentration of mercury, after the formation of a complex with 2-(5-bromo-2-pyridylazo)-5-(diethylamino)-phenol (5-Br-PADAP), and later analysis by electrothermal atomic absorption spectrometry (ETAAS) using polyethyleneglycolmono-p-nonyphenylether (PONPE 7.5) as surfactant. The chemical variables affecting the separation step were optimized. Under the optimum conditions, i.e, pH 8.5, cloud point temperature 80 degrees C, 5-Br-PADAP=4x10(-5) mol L(-1), PONPE 7.5=0.2%, sample volume=1.0 mL, an enhancement factor of 22-fold was reached. The lower limit of detection (LOD) obtained under the optimal conditions was 0.01 microg L(-1). The precision for 10 replicate determinations at 2.0 microg L(-1) Hg was 4.0% relative standard deviation (R.S.D.). The calibration graph using the pre-concentration system for mercury was linear with a correlation coefficient of 0.9994 at levels near the detection limits up to at least 16 microg L(-1). The method was successfully applied to the determination of mercury in biological samples and in certified reference material (QC METAL LL3).     

Liquid chromatography/tandem mass spectrometry (highly selective selected reaction monitoring) for the analysis of isopropylthioxanthone in packaged food

Isopropylthioxanthone (ITX), usually applied as a mixture of 2- and 4-isomers, is a common photo-initiator in UV inks used in paper- or plastic-based packaging materials. In this work a pentafluorophenylpropyl column (HS F5) has been used to achieve the chromatographic separation of the two isomers. A gradient elution with acetonitrile and a 25mM formic acid-ammonium formate at pH 3.75 are required to provide an Rs of 1.3 between the two compounds. The fragmentation pattern of ITX was studied using two mass analyzers, an ion trap (IT) (multi-stage fragmentation) and a triple quadrupole mass analyzer of hyperbolic rods (accurate mass (AM) measurement). The protonated molecule [M+H](+) observed in the mass spectrometry (MS) spectrum lost an isopropyl group, [M+H-C(3)H(6)](+). Later, this ion fragmented, yielding the radical ion [M+H-C(3)H(6)-CHO](+). The elemental composition of these product ions was confirmed by AM measurement. Electrospray ionization (ESI) was used as an ionization source to couple liquid chromatography (LC) to MS. Instrumental quality parameters of three acquisition modes provided by the triple quadrupole mass analyzer were studied and good run-to-run precision (relative standard deviation, RSD, lower than 10%) and limits of detection (LODs) down to 0.8pg injected in the LC-MS/MS system were obtained. Finally the LC-MS/MS method using H-SRM Q1 acquisition mode was used to analyze 2- and 4-ITX in a range of food samples. The use of highly selective selected reaction monitoring (H-SRM on Q1) resulted in improved selectivity without sensitivity loss.     

Liquid chromatography/multi-stage mass spectrometry of bisphenol A and its halogenated derivatives

We report a liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for analyzing bisphenol A (BPA) and its halogenated derivatives. Since only tetrachlorobisphenol A and tetrabromobisphenol A (TBBPA) are commercially available, mono-, di- and trichlorobisphenol A were synthesized and purified in order to be used as analytical standards. This family of compounds was studied using electrospray ionization and an ion trap mass analyzer in order to characterize the new compounds and to propose fragmentation pathways. Multi-stage mass spectrometry was used to confirm the genealogical relationship between the ions. Some product ions were traced from MS/MS to MS(4) and the labelled compounds BPA-d(16) and TBBPA-(13)C(12) were used to assign some product ion structures. In general, the deprotonated molecule [M--H](-) loses a methyl and/or a halogen group during both MS/MS and MS(3), while the neutral loss of CO was also observed in MS(3) spectra. We selected the most intense and characteristic MS/MS transitions for LC/MS/MS analysis. LC separation was performed in a reversed-phase column; methanol/water (no additives) was used as the mobile phase in gradient elution mode; and BPA-d(16) was chosen as the internal standard. Solid-phase extraction (SPE) was used to pre-concentrate and to clean up water samples. The SPE LC/MS/MS method allows BPA and its halogenated derivatives to be detected at a few parts-per-billion (ppb) in surface water.