Selenirene as an intermediate in the pyrolysis of 1,2,3-selenadiazole. Microwave spectra of 13C labelled selenoketene

Microwave Spectra of ¹³CH₂¹²C^80,78Se and ¹²CH₂¹³C^80,78Se (selenoketene) are recorded. B 5-¹³C-1,2,3-selenadiazole all four species are formed. The ¹³C scrambling may take place via a three-membered ring, selenirene, but as expected its microwave spectrum was not observed.     

ESCA: Eine Methode zur Bestimmung von Elementen und ihren Bindungszust?nden in der Oberfl?che von Festk?rpern

Zusammenfassung ESCA (nach Siegbahn: Electron Spectroscopy for Chemical Analysis) beruht auf dem Photoeffekt an inneren Atomorbitalen und ist auf alle Substanzen anwendbar. Die Routineuntersuchungen werden überwiegend an Festkörpern durchgeführt. Informationsträger sind Photo- und Augerelektronen, die verlustfrei von der Festkörperoberfläche emittiert werden. Auf Grund der Oberflächenempfindlichkeit gehört ESCA zu den mikrochemischen Verfahren. Die erfaßte Schichtdicke ist gegeben durch die Austrittstiefe der Photo- und Augerelektronen. Sie beträgt einige Monolagen, maximal 100 Å. Man kann davon ausgehen, daß bei einer untersuchten Fläche von 0,5 cm² und homogener Verteilung eines Elementes in den obersten 100 Å eine Konzentration von 1 % noch sicher nachgewiesen wird. Die Empfindlichkeitsunterschiede von Element zu Element liegen innerhalb eines Faktors 10. Matrixeffekte sind noch wesentlich geringer. Deshalb sind gute Voraussetzungen für eine quantitative Analyse gegeben. ESCA ist zwar nicht direkt ein Verfahren zur Spurenanalyse, wohl aber nach geeigneter Anreicherung. Der besondere Vorteil von ESCA liegt darin, daß man nicht nur Elemente nachweist, sondern an Hand von chemischen Verschiebungen, Linienaufspaltungen und Shake-up-Satelliten zuverlässige Aussagen über ihren Bindungszustand erhält. Weiterhin kann innerhalb des Bereiches der Austrittstiefe der Photoelektronen differenziert werden; dadurch ergeben sich Möglichkeiten zur zerstörungsfreien Aufnahme von Tiefenverteilungen von Elementen und Verbindungen. Zum Vordringen in tiefere Schichten kann ESCA mit abbauenden Verfahren gekoppelt werden. Nach Ionenbeschußabbau ist wegen ioneninduzierter Reaktionen in der Regel jedoch kein Verbindungsnachweis mehr möglich.

---

ESCA: A method for the determination of elements and their valence states in the surfaces of solids

Summary ESCA — or Electron Spectroscopy for Chemical Analysis (the term used by its inventor, Siegbahn)-is based on the effects of X-rays on the inner orbitals of atoms and is applicable to all substances. Routine ESCA investigations are carried out chiefly on solids. The information is provided by photo- and Auger electrons, which are emitted from the surfaces of solids without energy losses. Because of its surface sensitivity ESCA may be regarded as a microchemical method. The comprehended layer thickness is given by the escape depths of the photo- and Auger electrons. The range is from a few monolayers to 100 Å. As a rule, an element that is distributed homogeneously in the uppermost 100 Å of an area of 0.5 cm² can still be detected reliably at a concentration of 1%. The differences in sensitivity from element to element are within a factor of ten. Matrix effects are considerably smaller still. ESCA is therefore well suited for quantitative analysis. Though not directly suitable for trace analysis, ESCA can be used for that purpose after appropriate enrichment techniques have been applied. The special advantage of ESCA is that, in addition to detecting elements, it provides reliable information on their valence states by means of chemical shifts, line splitting and shake-up satellites. Apart from this, ESCA permits in-depth discrimination within the range of the escape depths of the photo electrons; it therefore provides opportunities for the non-destructive recording of the depth distributions of elements and compounds. For investigation of lower layers, ESCA can be combined with methods which involve the removal of outer layers. After removal of layers by ion bombardment, however, ion-induced reactions generally prevent the detection of compounds.

Hauptvortrag auf dem Internationalen Symposium für mikrochemische Arbeitsmethoden (ISM) Davos, Mai 1977     

Rearrangement of symmetrical dicubane clusters into topological analogues of the P cluster of nitrogenase: nature's choice?

Reaction schemes have been developed that lead to clusters having the topology of the PN cluster of nitrogenase. The single cubane clusters [(Tp)MFe3S4Cl3]z (M = Mo, z = 1-; M = V, z = 2-) react with PEt3 to give [(Tp)MFe3S4(PEt3)3]1+, which are reduced to the neutral edge-bridged double cubanes [(Tp)2M2Fe6S8(PEt3)4] with highly reduced (2[MFe3S4]1+) cores. Reaction of these clusters in acetonitrile with (Et4N)(HS) results in the formation of [(Tp)2Mo2Fe6S9(SH)2]3- and [(Tp)2V2Fe6S9(SH)2]4-. X-ray structures of the Et4N+ salts reveal the bridging pattern M2Fe6(mu2-S)2(mu3-S)6(mu6-S) in which two cuboidal MFe3(mu3-S)3 units share the common bridge atom mu6-S and are externally bridged by two mu2-S atoms. The M sites possess trigonal octahedral, and the Fe sites, distorted tetrahedral coordination. Hydrosulfide ligands and sulfide atoms simulate terminal cysteinate ligation and mu2 bridges, respectively, in the protein-bound cluster Fe8S7(mu2-SCys)2(SCys)4. The synthetic clusters have the same bridging pattern as the PN cluster and approach congruency with it. These clusters are the first molecular topological analogues of a PN cluster. Like the latter, they are substantially reduced (majority of Fe(II)).     

Inverted reactivity of aryllithium derivatives—IV: The reaction of thienyl-aryliodonium chlorides with nucleophiles

The product distribution in the reaction of aryl-thienyliodonium chlorides with some nucleophiles has been studied. CIDNP effects were observed in the reaction with alkoxides. The mechanism of the reaction is discussed.     

Radiochemical measurements of99Tc: Sources and environmental levels

A method of⁹⁹Tc determination has been developed based on sample decomposition followed by purification from interfering elements by hydroxide and oxide precipitations. Technetium is extracted by TBP and electrodeposited onto stainless steel discs from sodium hydroxide solution.⁹⁹Tc^m is used as yield monitor. The method has been applied to study releases from different sources and contents in different samples of atmospheric, terrestrial and marine origin. Results show enhanced⁹⁹Tc/¹³⁷Cs activity ratios for samples collected in recent years which are only expected to be contaminated from fallout from nuclear tests. Integrated fallout measurements did not reflect these enhanced ratios.     

Donor states in tellurium-doped silicon

Hall effect and conductivity measurements are performed on Te-doped silicon in the temperature range 30KT800K. A Hall equipment suited for high temperatures up to 800 K has been constructed. The temperature dependence of the free electron concentration is analyzed for Te-doped silicon including one double-donor and several monovalent donor species. A deep level with an electrical activation energy of 200 meV is determined from the saturation of the free electron concentration at temperatures above 400 K. This level represents the first ionization stage of the Te double-donor. The second ionization stage is estimated to have an activation energy of 440 meV. The maximum electrically active Te concentration obtained is 5×10¹⁶cm^–3. Three different shallow donor states are resolved in the low-temperature range. The concentrations of these shallow donors are partially sensitive to a subsequent heat-treatment.     

Substitution and oxidation reactions of bis(dithiolene)tungsten complexes of potential relevance to enzyme sites

Structurally characterized tungstoenzymes contain mononuclear active sites in which tungsten is coordinated by two pterin-dithiolene ligands and one or two additional ligands that have not been identified. In this and prior investigations (Sung, K.-M.; Holm, R. H. Inorg. Chem. 2000, 39, 1275; J. Am. Chem. Soc. 2001, 123, 1931), stable coordination units of bis(dithiolene)tungsten(IV,V,VI) complexes potentially related to enzyme sites have been sought by exploratory synthesis. In this work, additional members of the sets [WL(S2C2Me2)2](2-,-) and [WLL'(S2C2Me2)2](2-,-) have been prepared and structurally characterized. Tungsten(IV) complexes obtained by substitution are carbonyl displacement products of [W(CO)2(S2C2Me2)2] and include those with the groups W(IV)S (4), W(IV)(O2CPh) (5), and W(IV)(2-AdQ)(CO) (Q = S (6), Se (7); Ad = adamantyl). Those obtained by oxidation reactions contain the groups W(V)O (9), W(V)(QPh)2 (Q = S (10), Se (11)), W(VI)S(OPh) (12), and W(VI)O2 (14). The latter two complexes were obtained from W(IV) precursors using sulfur and oxygen atom transfer reactions, respectively. Complexes 4 and 9 are square pyramidal; 6, 7, 10, and 11 are distorted trigonal prismatic with cis ligands LL'; and 12 and 14 are distorted octahedral. Complexes 4, 10, and 11 support three-membered electron transfer series. Attempts to oxidize 4 to the W(V)S complex results in the formation of binuclear [W2(mu2-S)2(S2C2Me2)4](2-) having distorted octahedral coordination. The 21 known functional groups WL and WLL' in mononuclear bis(dithiolene) complexes prepared in this and prior investigations are tabulated. Of those with physiological-type ligands, it remains to be seen which (if any) of these ligation modes are displayed by enzyme sites.