J. Hormes

The thermal ageing of poly(3,4-ethylenedioxythiophene). An investigation by X-ray absorption and X-ray photoelectron spectroscopy

Abstract Oxidatively prepared poly(3,4-ethylenedioxythiophene tetrachloroferrate) (PEDT) has been characterized by X-ray absorption spectroscopy and XPS to study the thermal degradation of the polymer. XANES spectra were measured at the Electron Stretcher Accelerator ELSA in Bonn and compared with measurements made with the conventional XPS technique. These investigations show that after 48 h at 150°C due to baking and moisture irreversible structural changes in the polymer main chain occur. They were identified as a partial oxidation of the sulfur of the polymer backbone to the sulfate anion and the decomposition of the counter ion resulting in loss of conductivity. A possible degradation mechanism is presented.

Quantitative speciation of sulfur in bacterial sulfur globules: X-ray absorption spectroscopy reveals at least three different species of sulfur.

X-ray absorption near edge structure (XANES) spectroscopy at the sulfur K-edge was applied to probe the speciation of sulfur of metabolically different sulfur-accumulating bacteria in situ. Fitting the spectra using a least-square fitting routine XANES reveals at least three different forms of sulfur in bacterial sulfur globules. Cyclooctasulfur dominates in the sulfur globules of Beggiatoa alba and the very recently described giant bacterium Thiomargarita namibiensis. A second type of sulfur globules is present in Acidithiobacillus ferrooxidans: here the sulfur occurs as polythionates. In contrast, in purple and green sulfur bacteria the sulfur mainly consists of sulfur chains, irrespective of whether it is accumulated in globules inside or outside the cells. These results indicate that the speciation of sulfur in the sulfur globules reflects the different ecological and physiological properties of different metabolic groups of bacteria.

Theoretical and experimental investigations of the electronic circular dichroism and absorption spectra of bicyclic ketones

Abstract The absorption and circular dichroism (CD) spectra of the bicyclic ketones norcamphor, camphor and fenchone were measured in the gas phase in the energy range 3.5–8.5 eV (350–145 nm). The experimental results are compared with theoretical density functional calculations combined with a configuration interaction treatment employing extended AO basis sets. This approach gives a quantitative agreement between theory and experiment for the excitation energies and transition moments of four separated bands. On this basis a complete assignment of the excited singlet states involved could be made. In particular, an intense band found in the CD spectra at 7.0–7.5 eV is attributed to contributions from σ → π ∗ valence excited states which is a special feature of cyclic ketone. Relative energy shifts, the sign of the CD bands and signs inversions as a function of the methyl group substitution pattern in the three molecules are discussed. A simple model which partitions the bicyclic structures into an inherently chiral cyclopentanone moiety and perturbing substituents is useful in this respect.

Experimental and quantum-theoretical investigation of the circular dichroism spectrum of R-methyloxirane

Abstract Methyloxirane, as a prototype for other substituted oxirane systems, was chosen for spectroscopic and theoretical study of circular dichroism. Vacuum UV absorption and CD spectra are reported, as well as ab initio multireference configuration interaction calculations for the rotatory strengths of the first three CD absorption bands. Good agreement between theory and experiment is obtained.

Interaction between core and protection shell of N(butyl)4Cl- and N(octyl)4Cl-stabilized Pd colloids

Abstract We present X-ray absorption near edge structure (XANES) measurements on N(butyl)4Cl- and N(octyl)4Cl-stabilized Pd colloids at both the Pd LIII- and the Cl K-edge. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (HeI) results for these colloids, deposited on silica substrates, are also shown. The results provide detailed insight into the mechanism of bonding between the protection shell and the colloidal core. Both XANES and MIES suggest that the chlorine is present on the inside of the protection shell, located between palladium core and N(alkyl)4 groups forming the protection shell. Moreover, the XANES results suggest a dependence of the equilibrium position of the chlorine between the metal core and the alkyl chain on the length of the alkyl chains. The possible motivation for such an effect is discussed on the basis of different models. MIES, in addition, provides information on the thermal stability of the shell-stabilized Pd colloids.

Surface engineering of Co and FeCo nanoparticles for biomedical application

Monodisperse Co, Fe, and FeCo nanoparticles are prepared via thermal decomposition of metal carbonyls in the presence of aluminium alkyls, yielding air-stable magnetic metal nanoparticles after surface passivation. The particles are characterized by electron microscopy (SEM, TEM, ESI), electron spectroscopy (MIES, UPS, and XPS) and x-ray absorption spectroscopy (EXAFS). The particles are peptized by surfactants to form stable magnetic fluids in various organic media and water, exhibiting a high volume concentration and a high saturation magnetization. In view of potential biomedical applications of the particles, several procedures for surface modification are presented, including peptization by functional organic molecules, silanization, and in situ polymerization.

Sulfur K-shell photoabsorption spectroscopy of the sulfanes RSnR, n = 2–4

Abstract X-ray absorption near edge structure (XANES) measurements were carried out at the sulfur K-edge of nine sulfanes with the structure RSnR, n = 2–4. The discrete part of the XANES spectra of these sulfanes is expected to be quite similar under the change of the hydrocarbon substituents R. However, significant differences were observed in the energy splitting of the pre-edge resonances as well as in the relative intensities of these features. A linear correlation is observed between the splitting of the sulfur 1 s → σ ∗ ( SC ) and 1 s → σ ∗ ( SS ) transitions and the SC bond length. Similar correlations are expected for the energy splitting and the SC bond enthalpy and the difference in electronegativity between the sulfur atom and the substituent R. We have carried out MS-X α calculations on the CSS fragment to support the proposed assignment and the parameter dependence of the resonances mentioned above.

Experimental and theoretical study of the circular dichroism spectra of oxa- and thia-[2.2]metacyclophane

Abstract The excited electronic states of (−)-(M)-1-oxa [2.2] metacyclophane and (−)-(M)-1-thia[2.2] metacyclophane are discussed in the light of experimental UV/VUV optical absorption and circular dichroism (CD) spectra and multireference configuration interaction (MRD-CI) calculations of excitation energies and rotatory strengths. As one-particle basis, semi-empirical AM1 and MNDOC wavefunctions expressed in terms of localized molecular orbitals (LMO) are used. Good agreement between experimental and calculated CD spectra is obtained by correlating only a small portion of the 80 valence electrons of the compounds. The dominant CD transitions of the oxa [2.2] metacyclophane can be characterized as separate and coupled ππ* transitions localized in the aromatic rings. The CD spectrum of the thia [2.2] metacyclophane is mainly described by valence-type excitations involving the lone-pair orbital of the sulphur atom (3p) and/or anti-bonding σ* MOs of CS bonds; localized ππ* transitions are of lower intensity.

Structure analyses of Ba-silicate glasses

Abstract The structures of four different Ba-silicate glasses were analysed applying a broad spectrum of diffraction and spectroscopic methods, atomic force microscopy and computer simulations. This collaborative study offers new insights into the influence of the network modifier BaO. 3 at.% carbon were incorporated into two of the glasses with the intention to obtain additional information about the effect of partial substitution of carbon for oxygen on the considered structures. A structure model for Ba-silicate glasses with compositions close to BaSi 2 O 5 is discussed that differs with respect to other models proposed in previous studies. We propose that [SiO 4 ] tetrahedra do not exclusively form six-membered rings, but also considerable numbers of smaller rings arranged in (slightly) folded layers which alternate with barium layers and/or barium chains. Additionally, the occurrence of tetrahedral chains, possibly isolated rings and of regions in which the [SiO 4 ] tetrahedra form a silica-like network is assumed.

Vanadium K-edge absorption spectrum of bromoperoxidase from Ascophyllum nodosum

With synchrotron radiation from the Bonn 2.5 GeV synchrotron, high-resolution absorption spectra have been measured at the vanadium K-edge of bromoperoxidase from the marine brown alga Ascophyllum nodosum and several model compounds. The near-edge structure (XANES) of these spectra was used to determine the charge state and the coordination geometry around the vanadium atom. For the active enzyme a coordination charge of 2.7 was found which is compatible with a formal valence of +5, assuming coordination by atoms with a high electronegativity such as oxygen or nitrogen. For the reduced enzyme the coordination charge value of 2.15 indicates the reduction of the valency by 1 unit. Our results suggest that the coordination sphere of the vanadium atom in the native enzyme consists of at least seven oxygen atoms in a distorted octahedral environment with an average bond length of about 2 A. Through the reduction process, the coordination sphere of the vanadium atom changes with a simultaneous decrease of the coordination cage. These results agree with those deduced from previous EPR and 51V-NMR measurements.