A simple method for the characterization of OHO-hydrogen bonds by H-solid state NMR spectroscopy

A set of OHO hydrogen bonded systems with known neutron diffraction structure has been studied by fast ¹H-MAS echo spectroscopy. It is shown that the application of a simple rotor synchronized echo sequence combined with fast MAS allows a faithful determination of the chemical shift of the proton in the hydrogen bond. Employing the empirical valence bond order model, the experimental ¹H chemical shifts of the hydrogen bonded protons are correlated to the hydrogen bond geometries. The resulting correlation between the proton chemical shift and the deviation of the proton from the center of the hydrogen bond covers a broad range of substances. Deviations from the correlation curve, which are observed in certain systems with strong hydrogen bonds, are explained in terms of proton tautomerism or delocalization in low-barrier hydrogen bonds. These deviations are a highly diagnostic tool to select potential candidates for further experimental and theoretical studies. Thus, the combination of the ¹H-MAS echo sequence with the correlation curve yields a simple and versatile tool for the structural analysis of OHO hydrogen bonds.     

C-Nitroso compounds—XXXV : Reaction of organometallic compounds with 1-chloro-1-nitroso-2,2,6,6-tetramethylcyclohexane

Reaction of Grignard reagents and organolithium compounds (RM) with the congested 1-chloro-1-nitroso-2,2,6,6-tetramethylcyclohexane 1 leads to the formation of significant amounts of the reduction product 2,2,6,6-tetramethylcyclohexanone oxime 3 (61–90%) together with the corresponding oxime O-R ether 4 (0–11%). Attack on nitrogen is unimportant as shown by very low yields of nitrone. Formation of the products is rationalised with a pathway involving transfer of an electron from RM to 1. This leads—after separation of MCI—to a radical pair consisting of R and the relatively stable iminoxy radical 2 (Schemes 1 and 2). Combination of these radicals explains formation of oxime ether 4 and nitrone 5, while reaction of iminoxy radical 2 with excess of RM can give oxime 3. Reactive radicals R (i.e. Me, Ph, and to a minor extent n-Bu) are furthermore capable of abstracting hydrogen from the solvent (diethyl ether, toluene, or cumene), and the solvent derived radicals can also combine with 2 on oxygen, under formation of oxime ether (26% of 6a). The corresponding benzyl- and cumyl ethers 6b and 6c are only formed in trace amounts because dimerisation of benzyl radicals (7%) and cumyl radicals (22%) is favoured.     

Electron impact dissociation of H2O: emission cross sections for OH*, OH+*, H*, and H2O+* fragments

The optical emission spectrum in the near ultraviolet and visible following electron impact on H₂O was studied in a crossed-beam and a static gas-target experiment. Emissions of H*, OH*, OH⁺*, and H₂O⁺* fragments were detected and absolute emission cross sections for the different fragments were determined. A nonthermal rotational population was observed for the diatomic fragments which gives insight into the dissociation process. Further conclusions on the dissociation mechanism are possible based on appearance potentials and the shape of the emission cross sections as a function of impact energy.     

Die spezifische Wärme des Polyvinylchlorids

Zusammenfassung Die spezifische Wärme verschiedener handelsüblicher Polyvinylchlorid-Sorten (Suspensions- und Emulsions-PVC, schlagfestes PVC und ein Vinylchlorid-Vinylacetat-Copolymerisat) wurde im Temperaturbereich 20 (bzw. –50) bis 140 °C mit einem adiabatischen Kalorimeter gemessen. Besondere Aufmerksamkeit wurde dem Einfluß der thermischen Vorgeschichte gewidmet. Messungen an getemperten Proben ergaben — in Übereinstimmung mit den Ergebnissen anderer Autoren —einfache Kurvenzüge mit einem Steilanstieg der spezifischen Wärme im Einfriergebiet. Untersuchungen an abgeschreckten Proben ließen zu Beginn des Einfrierbereiches Minima der spezifischen Wärme infolge Enthalpierelaxation erkennen. Oberhalb des Einfrierbereichs zeigten sich Kristallisationserscheinungen mit Wärmetönungen von etwa –1,3 cal/g (exotherm). Hieraus wurde der kristalline Anteil des Polyvinylchlorids zu rund 3% abgeschätzt. Der Schmelzpunkt der PVC-Kristallite wurde differentialthermoanalytisch zu 156 bzw. 170 °C gefunden. Das schlagfeste PVC ließ das Schmelzen einer Spur Polyäthylen zwischen 102 und 125 °C erkennen. Die kalorimetrisch bestimmten Einfriertemperaturen stimmen mit dilatometrisch gemessenen — gleiche thermische Vorbehandlung vorausgesetzt — überein.

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Summary The specific heat of some commercially available samples of polyvinyl chloride (suspension PVC, emulsion PVC, high impact PVC, and a copolymerisate of vinylchloride and vinylacetate) was measured in the temperature range from 20 (or –50) to 140 °C, using an adiabatic calorimeter. Special attention was paid to the influence of thermal history of the samples. Investigations of annealed samples gave simple curves with a steep slope in the glass transition range, in agreement with the results of other authors. Measurements with samples quenched in ice water showed specific heat curves with a minimum at the beginning of the glass transition range caused by enthalpy relaxation. Above the glass transition range crystallization occurred accompanied by heat effects of about –1,3 cal/g (exothermal). From this the fraction of crystalline PVC was estimated to be about 3%. The melting point of the PVC crystallites as determined by differential thermal analysis was 156 or 170 °C. With high impact PVC the melting of traces of polyethylene was observed between 102 and 125 °C. The glass transition temperatures as determined by calorimetry agreed with those determined by dilatometric measurements, provided thermal pretreatment being equal in both cases.

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Mit 14 Abbildungen und 5 Tabellen     

Low-level radiometric detection in LC using solvent segmentation and an effluent storage loop

Summary In this study a new approach is presented for on-line radiometric detection in reversed phase LC of medium to low polarity compounds labelled with¹⁴C. The test compounds,¹⁴C-carbaryl and¹⁴C-parathion, are extracted post-column into a non-water miscible liquid scintillation cocktail. The segmented two-phase system formed is introduced into the beta-detector without phase separation and collected in a capillary storage tube. After completion of the LC separation and detection process, the direction of the flow in the storage system is reversed and the segmented contents of the loop led at lower flow-rates through the beta-detector again. An enhanced signal, corresponding to the increase in counting time is obtained without measurable peak broadening. The lowest possible detection limit of the system is 9 counts per peak corresponding to subnanogram quantities of tested pesticides. Calibration curves are linear over at least 2 orders of magnitude and have the expected theoretical slopes. The reproducibility of the system is better than 4 % rel. S.D. An application to a recovery study of parathion shows the practical potential of this technique. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Interaction of the mutagen ethidium bromide with DNA, using a carbon paste electrode and a hanging mercury drop electrode

The interaction of ethidium bromide (2,7-diamino-10-ethyl-9-phenylphenanthridinium bromide; EB) with double stranded (ds) calf thymus DNA and thermally denatured single stranded (ss) DNA was studied in solution and at the electrode surface by means of transfer voltammetry using a carbon paste electrode (CPE) as working electrode in 0.2 M acetate buffer, pH 5.0. As a result of intercalation of this dye between the base pairs of dsDNA, the characteristic peak of dsDNA, due to the oxidation of guanine residues, decreased and after a particular concentration of EB a new peak at +0.81 V appeared, probably due to the formation of a complex between dsDNA and EB. The non-intercalated EB gives another peak, but at an increased concentration of the dye. A similar behaviour was observed during the interaction of the dye with ssDNA.Furthermore, the interaction of EB with ds, ss and supercoiled (sc) DNA was studied at the hanging mercury drop electrode (HMDE) surface by means of alternating current voltammetry in 0.3 M NaCl and 50 mM sodium phosphate buffer (pH 8.5) as supporting electrolyte. dsDNA yields a smaller peak at −1.42 V (peak III) compared to the one yielded by ssDNA, since the latter is a relaxed and more accessible form. By addition of EB into the buffer solution an increase of peak III was observed in the dsDNA form as well as in ssDNA resulting from their interaction with EB. Furthermore, the appearance of peak III in covalently closed circular scDNA after exposure to increasing concentrations of EB is a result of the introduction of ‘free ends’ in DNA affecting its structural integrity.     

Group separation of organohalogenated compounds by means of comprehensive two-dimensional gas chromatography

Separations of 12 compound classes, polychlorinated biphenyls (PCBs), diphenyl ethers (PCDEs), naphthalenes (PCNs), dibenzothiophenes (PCDTs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), terphenyls (PCTs) and alkanes (PCAs), toxaphene, organohalogenated pesticides (OCPs), and polybrominated biphenyls (PBBs) and diphenyl ethers (PBDEs) by comprehensive two-dimensional gas chromatography were evaluated. Five column combinations, DB-1 x 007-210, DB-1 x HT-8, DB-1 x LC-50, DB-1 x 007-65HT and DB-1 x VF-23ms were used to study, primarily, group-type separations, but attention was devoted also to within-class separation, especially for those classes which were not addressed in much detail before, the PCNs, OCPs, PBBs and PCTs. The DB-1 x 007-210 column set did not offer any extra separation compared to one-dimensional GC. For the DB-1 x HT-8 column combination, the useful principle of congener separation on the basis of number of halogen substituents in a molecule was confirmed (PCBs, toxaphene) and extended (PCTs, PBDEs). No practically useful group-type separation was observed for this column combination. The DB-1 x LC-50 set provides group separation based on planarity: planar compounds such as PCDDs, PCDFs, PCDTs and PCNs are much more retained than, and therefore separated from, non-planar analytes. Within the classes of PCBs, PBBs and PCTs highly useful separation of planar from non-planar compounds was also observed. The DB-1 x 007-65HT column set effectively separates PCAs and PBDEs from all other compound classes, and provides a good separation of brominated and chlorinated analogue classes from each other. This column set was the most efficient one for within-class separation of OCPs and PCNs. Finally, DB-1 x VF-23ms yields excellent within-class separations, especially of non-aromatic compounds, viz. OCPs, toxaphene and PCAs. No group separation was observed here. The applicability of the approach was demonstrated for a sediment extract and a dust extract. In the sediment extract, PCDDs, PCDFs, PCAs and PCNs were identified and their efficient separation was achieved. In the dust sample, separation of PCAs and PBDEs was achieved and several new PBDE congeners were identified.     

Hydrogen isotope effects in electron donor-acceptor systems

The charge-transfer (CT) transition of complexes involving alkylated aromatic electron donors shifts hypsochromically upon benzylic deuteration. This is attributed to hyperconjugative effects.     

Synthesis of new triphenodithiazine- and indolocarbazolediones of biological interest

Tetrabromo-p-benzoquinone reacted with excess aromatic amines to give 2,5-dirylamino-3,6-dibromo-p-benzoquinones. The latter molecules on heating with sodium sulfide in alcohol in the presence of air gave triphenodithiazinediones. Heating with copper powder in nitrobenzene transformed these compounds into the respective indolocarbazolediones. Comparative antimicrobial and antifungal activities of the studied compounds were determined and discussed.     

Untersuchung der Sekundärstruktur von anodischen Deckschichten auf Aluminium durch Farbstoffaufnahmen

Zusammenfassung Der zeitliche Verlauf der Farbstoffaufnahme an den beiden aufeinanderliegenden Al₂O₃-Schichten bei anodisch oxydiertem Aluminium wird in beiden F?llen durch die „Anlaufparabel“ dargestellt. Die Gültigkeit dieser Gleichung best?tigt die Bildung eines Lackes zwischen dem benutzten Farbstoff und dem Aluminiumoxyd bei beiden Schichten. Die Geschwindigkeitskonstante für die erste Schicht desx ₁-Al₂O₃ ist der Stromdichte proportional, mit welcher die Oxydation vorgenommen wurde; für die zweite Schicht desx ₂-Al₂O₃ ist dasK″ unabh?ngig von der Stromdichte. In dem Diagramm desK″ gegen die Stromdichte zeigt sich ein Sprung in den Eigenschaften der beiden Schichten bei 8600 Coul/dm² (9,55 A/dm² · 15 · 60), d.h. an demselben Punkt, an dem er sich auch in der Arbeit (1) gezeigt hatte. Diese Lage des Sprunges best?tigt die Messungen und die Ergebnisse der beiden Arbeiten. Au?erdem wurde so von neuem bewiesen, da? an der Grundschicht anliegendesx ₁-Al₂O₃ por?s ist und infolgedessen die Geschwindigkeit der Farbstoffaufnahme von seiner Gesamtoberfl?che abh?ngt, d. h. von seiner Dicke (1) und deshalb von der Stromdichte. Im Gegensatz dazu ist die daraufliegende Schicht vonx ₂-Al₂O₃ weniger por?s; infolgedessen ist die Geschwindigkeit der Farbstoffaufnahme unabh?ngig von seiner Dicke (1) und von der Stromdichte, mit der die Oxydation durchgeführt worden ist (1). Die Gültigkeit der Arrheniusschen Gleichung für beide Schichten erlaubt auch die Bestimmung der Aktivierungsenergie, deren Wert (identisch für beide Schichten), auf Grund der Theorie der Fehlordnung und der Gültigkeit der Anlaufparabel, den Ablauf einer Platzwechselreaktion beweist. Durch die vorliegende Arbeit im Zusammenhang mit der Arbeit (1) wurde das Bild von der anodischen Oxydation des Aluminiums und von der Farbstoffaufnahme vervollst?ndigt, sowohl vom Standpunkt der Sekund?rstruktur des Oxyds (hier wurde zum erstenmal die Anwesenheit von zwei übereinanderliegenden Oxydschichten in der Zone der Hauptschicht mit deutlich verschiedenen Eigenschaften bewiesen), als auch vom Standpunkt der Reaktionskinetik aus (hier wurde der Reaktionsmechanismus aufgedeckt).