Synthesis, structure, and spectroscopic characterization of [H(8-n)Rh22(CO)35](n-) (n = 4, 5) and [H2Rh13(CO)24{Cu(MeCN)}2](-) clusters: assessment of CV and DPV as techniques to circumstantiate the presence of elusive hydride atoms

The previously ill-characterized [H(x)Rh(22)(CO)(35)](4-/5-) carbonyl cluster has been obtained as a byproduct of the synthesis of [H(3)Rh(13)(CO)(24)](2-) and effectively separated by metathesis of their sodium salts with [NEt(4)]Cl. Although the yields are modest and never exceed 10-15% (based on Rh), this procedure affords spectroscopically pure [H(3)Rh(22)(CO)(35)](5-) anion. Formation of the latter in mixture with other Rh clusters was also observed by electrospray ionization-mass spectrometry (ESI-MS) in the oxidation of [H(2)Rh(13)(CO)(24)](3-) with Cu(2+) salts. The recovery of further amounts of [H(3)Rh(22)(CO)(35)](5-) was hampered by too similar solubility of the salts composing the mixture. Conversely, the reaction in CH(3)CN of [H(2)Rh(13)(CO)(24)](3-) with [Cu(MeCN)(4)](+)[BF(4)](-) leads to the [H(2)Rh(13)(CO)(24){Cu(MeCN)}(2)](-) bimetallic cluster. The X-ray crystal structures of [H(4)Rh(22)(CO)(35)](4-), [H(3)Rh(22)(CO)(35)](5-), and [H(2)Rh(13)(CO)(24){Cu(MeCN)}(2)](-) are reported. From a formal point of view, the metal frame of the former two species can be derived by interpenetration along two orthogonal axes of two moieties displaying the structure of the latter. The availability of [H(8-n)Rh(22)(CO)(35)](n-) salts prompted their detailed chemical, spectroscopic, and electrochemical characterization. The presence of hydride atoms has been directly proved both by ESI-MS and (1)H NMR. Moreover, both [H(4)Rh(22)(CO)(35)](4-) and [H(3)Rh(22)(CO)(35)](5-) undergo distinctive electrochemically reversible redox changes. This allows to assess electrochemical studies as indisputable though circumstantial evidence of the presence of (1)H NMR-silent hydride atoms in isostructural anions of different charge.     

Mixed conductivity in nanocrystalline highly acceptor doped cerium oxide thin films under oxidizing conditions

We report on the investigation of 10 mol% gadolinium-doped cerium oxide thin films of various microstructures prepared by pulsed laser deposition. Depending on substrate, growth conditions and hence microstructure, the electric conductivity values vary considerably by several orders of magnitude. Remarkably, in the sample with the highest grain boundary density, we even have evidence of substantial electronic conductance under oxidizing conditions despite the large acceptor level. This possibly surprising result can be explained by an increased space charge potential at the grain boundaries in combination with the small grain size of 10 nm that leads to an enrichment of excess electrons while the ion conduction is simultaneously blocked by vacancy-depleted regions.     

Use of high-efficiency packed columns for gas-solid chromatography : V. Micropacked columns

Micropacked columns are constructed using graphitized carbon black as a support for gas-liquid-solid chromatography. Their effectiveness is evaluated in terms of Van Deemter plots, flow-rates, inlet pressure and column capacity. It is shown that such columns, by choosing an appropriate liquid phase, can be used for a wide range of temperatures and types of compound. Analytical applications connected with the analysis of complex organic mixtures in water are discussed. The problems of the analysis of air pollutants are also considered.     

Physical and chemical properties of tussah silk fibroin films

Physical and chemical structure, as well as thermal behavior of solution-cast regenerated films, prepared from tussah (Antheraea pernyi) silk fibroin, were compared with those of solution-cast native films, in order to ascertain whether treatment (degumming, dissolution) used for preparation affected their properties. Regenerated fibroin films exhibited a higher thermal stability than native ones, as shown by differential scanning calorimetry, thermomechanical analysis, and dynamic mechanical behavior. Glass transition temperature and other relevant thermal transitions of the regenerated silk specimen shifted to higher temperatures compared with those of native specimen. Molecular conformation and crystalline structure did not show significant differences between the two kinds of silk films. Amino acid composition and molecular weight, however, distribution changed markedly after dissolving tussah silk fibroin fiber in concentrated LiSCN in polypeptide size was the main features for the regenerated silk fibroin. © 1994 John Wiley & Sons, Inc.     

Electronic properties of b.c.c. transition metal solid solutions. a study of the Nr−Nb system

Summary Calculations of electron densities of states and of soft—X-ray absorption spectra in Zr−Nb alloys have been performed in the frame of the coherent potential approximation in its multiple-scattering version (KKR-CAP). It is shown that a lot of care is needed in applying the simple rigid-band model to these systems, in order to explain experimental data. Results are also diseussed in connection with existing experimental data and good agreement is found.

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Riassunto Sono stati eseguiti calcoli di densità di stati elettronici e di spettri di assorbimento di raggi X molli nella lega Zr−Nb a varie concentrazioni, nell'àmbito dell'approssimazione di potenziale coerente nella sua versione di scattering multiplo (KKR-CPA). Si mostra che è necessaria molta attenzione per poter interpretare dati sperimentali come semplice modello a bande rigide. I risultati sono inoltre, discussi in connessione con, i dati sperimentali esistenti con i quali si riscontra un buon accordo.

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Резюме в рамках приближения когерентного потенциала и его модификации для многократного рассеяния проводится вычисление плотностей электронных состояний и спектров поглощения мягких рентгеновских лучей в Zn−Nb сплавах. Показывается, что необходимо проявлять особое внимание при интерпретации экспериментальных данных с помощью простой модели недеформируемых зон. Полученные результаты обсуждаются в связи с имеющимися экспериментальными данными и овнаружено хорошее согласие.

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Work also sponsored by Assessorato Industria della Regione Siciliana.