Dealing with a local heating effect when measuring catalytic solids in a reactor with Raman spectroscopy

In continuation of our previous work on the applicability of the G(R(infinity)) correction factor for the quantification of Raman spectra of coke during propane dehydrogenation experiments (Phys. Chem. Chem. Phys., 2005, 7, 211), research has been carried out on the potential of this correction factor for the quantification of supported metal oxides during reduction experiments. For this purpose, supported chromium oxide catalysts have been studied by combined in situ Raman and UV-Vis spectroscopy during temperature programmed reduction experiments with hydrogen as reducing agent. The goal was to quantify on-line the amount of Cr(6+) in a reactor based on the measured in situ Raman spectra. During these experiments, a significant temperature effect was observed, which has been investigated in more detail with a thermal imaging technique. The results revealed a temperature 'on the spot' that can exceed 100 degrees C. It implies that Raman spectroscopy can have a considerable effect on the local reaction conditions and explains observed inconsistencies between the in situ UV-Vis and Raman data. In order to minimize this heating effect, reduction of the laser power, mathematical matching of the spectroscopic data, a different cell design and a change in reaction conditions has been evaluated. It is demonstrated that increasing the reactor temperature is the most feasible method to solve the heating problem. Next, it allows the application of in situ Raman spectroscopy in a reliable quantitative way without the need of an internal standard.     

Cooperativity in ionic liquids

Cooperativity in ionic liquids is investigated by means of static quantum chemical calculations. Larger clusters of the dimethylimidazolium cation paired with a chloride anion are calculated within density functional theory combined with gradient corrected functionals. Tests of the monomer unit show that density functional theory performs reasonably well. Linear chain and ring aggregates have been considered and geometries are found to be comparable with liquid phase structures. Cooperative effects occur when the total energy of the oligomer differs from a simple sum of monomer energies. Cooperative effects have been found in the structural motifs examined. A systematic study of linear chains of increasing length (up to nine monomer units) has shown that cooperativity plays a more important role than expected and is stronger than in water. The Cl...H distance of the chloride to the most acidic proton increases with an increasing number of monomer units. The average bond distance approaches 218.9 pm asymptotically. The dipole moment grows almost linearly and the dipole moment per monomer unit reaches the asymptotic value of 16.3 D. The charge on the chloride atoms decreases with an increasing chain length. In order to detect local hydrogen bonding in the clusters a new parametrization of the shared-electron number method is introduced. We find decreasing hydrogen bond energies with an increasing cluster size for both the first hydrogen bond to the most acidic proton and the average hydrogen bond.     

Ring-Closure Reactions. 13.1 A Convenient Synthesis of Many-Membered Thiacycloalkanes

Interaction of α,ω-dihaloalkanes with sulfide ion (eq. 1) is a common route for the synthesis in good yield of 5- and 6-membered polymethylene sulfides.     

6-Magnesiated purines: preparation and reaction with aldehydes

[reaction: see text] Halogen-metal exchange reaction of 9-benzyl-6-iodopurine with iPrMgCl in toluene at -80 degrees C proceeds almost quantitatively. Such a purine-derived Grignard reagent reacts selectively with aldehydes in toluene, giving the corresponding alcohols in 25-62% yield, while other functional groups such as ketones, esters, and nitriles do not react under these conditions. The reaction can be extended to protected 6-iodopurine ribonucleoside.     

Automatic determination of phylloquinone in vegetables and fruits using on-line photochemical reduction and fluorescence detection via solid phase extraction and flow injection

A very simple, rapid and highly sensitive flow injection fluorimetric method was developed for the determination of phylloquinone. The assay was based on the on-line reduction of phylloquinone in dodecylsulfate micelles after irradiation with UV light. The micellar medium enhanced the fluorescence and stability of the reduced phylloquinone. Under optimum experimental conditions, the range of application of the technique was between 0.09 and 45.0 μg mL⁻¹ and the detection limit was 0.05 μg mL⁻¹. The sample throughput was 90 injections per hour. The reliability of the method for the routine analysis of phylloquinone in vegetables and fruits is demonstrated. Extractions were made with hexane, and an automated solid phase extraction system was used to purify the sample extracts prior to injection into the flow injection manifold.     

Proton transfer reaction ion trap mass spectrometer

Proton transfer reaction mass spectrometry is a relatively new field that has attracted a great deal of interest in the last few years. This technique uses H(3)O(+) as a chemical ionization (CI) reagent to measure volatile organic compounds (VOCs) in the parts per billion by volume (ppbv) to parts per trillion by volume (pptv) range. Mass spectra acquired with a proton transfer reaction mass spectrometer (PTR-MS) are simple because proton transfer chemical ionization is "soft" and results in little or no fragmentation. Unfortunately, peak identification can still be difficult due to isobaric interferences. A possible solution to this problem is to couple the PTR drift tube to an ion trap mass spectrometer (ITMS). The use of an ITMS is appealing because of its ability to perform MS/MS and possibly distinguish between isomers and other isobars. Additionally, the ITMS duty cycle is much higher than that of a linear quadrupole so faster data acquisition rates are possible that will allow for detection of multiple compounds. Here we present the first results from a proton transfer reaction ion trap mass spectrometer (PTR-ITMS). The aim of this study was to investigate ion injection and storage efficiency of a simple prototype instrument in order to estimate possible detection limits of a second-generation instrument. Using this prototype a detection limit of 100 ppbv was demonstrated. Modifications are suggested that will enable further reduction in detection limits to the low-ppbv to high-pptv range. Furthermore, the applicability of MS/MS in differentiating between isobaric species was determined. MS/MS spectra of the isobaric compounds methyl vinyl ketone (MVK) and methacrolein (MACR) are presented and show fragments of different mass making differentiation possible, even when a mixture of both species is present in the same sample. However, MS/MS spectra of acetone and propanal produce fragments with the same molecular masses but with different intensity ratios. This allows quantitative distinction only if one species is predominant. Fragmentation mechanisms are proposed to explain the results.     

Sonic spray ionization applied to liquid chromatography/mass spectrometry analysis of endocrine-disrupting chemicals in environmental water samples

A sonic spray ionization liquid chromatography/mass spectrometry (LC/SSI-MS) procedure combined with off-line solid-phase extraction was optimized for the analysis of 20 endocrine-disrupting chemicals (EDCs) in water samples. Method development included a comparison of the novel sonic spray ionization (SSI) with more traditional ion sources, i.e. pneumatically assisted electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). It was demonstrated that SSI and ESI spectra were very similar, but were more prone to the formation of solvent cluster ions as compared with APCI spectra. This phenomenon was most prominent for SSI and resulted in an increased chemical background in full-scan mass spectra. However, this chemical noise did not affect the overall sensitivity of SSI and ESI. After optimization of LC and MS parameters, the LC/SSI-MS method was validated. Recoveries ranged from 76.3 up to 113.4% for all compounds. Limits of detection (LOD) and quantitation (LOQ) were established between 3.0 and 11.5 ng/L and 9.9 and 38.0 ng/L, respectively. Within-day (n = 5) and between-day (n = 5) reproducibility were investigated at three levels and ranged from 3.3-16.5% and 7.6-19.2%, respectively. Eight-point calibration curves were established and showed linearity for all compounds (r(2) > 0.987) over a linear dynamic range of 10-10 000 ng/L.     

Synthesis of HyBeacons and dual-labelled probes containing 2'-fluorescent groups for use in genetic analysis

An FMOC-protected 2'-hydroxyethyl uridine phosphoramidite has been used to synthesise fluorescein-labelled HyBeacon probes and "FAM-ROX" dual-labelled fluorogenic oligonucleotides.     

Diels-Alder reactions of pyridinone o-quinodimethanes generated from substituted sulfolene[3,4-c]pyridin-4(1H)-ones

1-Benzyl-3-(bromomethyl)-2(1H)-pyrazinone was converted to [3,4-c] sulfolene pyridinone 8a and further (1- or 3-) substituted derivatives having a dienophilic side chain on the sulfolene ring. Thermolytic extrusion of sulfur dioxide from o-QDM precursor 8 led to generation of 3,4-dimethylene-2(1H)-pyrazinone 9, which was reacted in situ with various dienophiles. Thermolysis of the substituted precursors resulted in intramolecular cycloaddition of the corresponding o-QDM intermediates