Direct contact versus solvent-shared ion pairs in NiCl2 electrolytes monitored by multiplet effects at Ni(II) L edge X-ray absorption

We investigate the local electronic structure in aqueous NiCl2 electrolytes by Ni L edge X-ray absorption spectroscopy. The experimental findings are interpreted in conjunction with multiplet calculations of the electronic structure and the resulting spectral shape. In contrast to the situation in the solid, the electronic structure in the electrolyte reflects the absence of direct contact Ni-Cl ion pairs. We observe a systematic change of the intensity ratio of singlet- and triplet-related spectral features as a function of electrolyte concentration. These changes can be described theoretically by a change in the weight of transition matrix contributions with different symmetries. We interpret these findings as being due to progressive distortions of the local symmetry induced by solvent-shared ion pairs.     

New details in the polarized spectrum of naphthalene by means of linear dichroism studies in oriented polymer matrices

Linear dichroism (LD) spectra are presented for naphthalene oriented in stretched polyethylene and polypropylene matrices at 77 K and 296 K. From the calculated spectrum LD(λ)/A(λ), where A(λ) is the corrected absorbance spectrum of the sample by unpolarized light, orientational parameters are calculated and component spectra, 235–315 nm, are resolved corresponding to polarization parallel to the long (B_3u = x) and the short (B_2u = y) axes in the molecular plane (D_2h). The orientational parameters indicate different orientational mechanisms in polyethylene and polypropylene, but the resolving procedure yields mainly identical component spectra. It is suggested that the polarization (B_3u) predominating in the 245–275 nm region isdue to a B_1g vibronic perturbation of the ¹B_2u state.     

Probing weak molecular orbital interactions in non-conjugated diene molecules by means of near-edge X-ray absorption spectroscopy

Carbon and oxygen near-edge X-ray absorption fine structure (NEXAFS) spectra of 1,4-cyclohexadiene, p-benzoquinone, norbornadiene, norbornadienone, and cis-cis-[4,4,2]propella-3,8-diene-11,12-dione were calculated by means of Hartree-Fock and hybrid density functional theory using the static-exchange (STEX) approximation. The NEXAFS spectra are used as a probe to identify weak molecular interactions between the two non-conjugated ethylenic pi* orbitals present in these molecules. We show that the X-ray absorption spectrum of 1,4-cyclohexadiene exhibits some particular spectral structures in the discrete energy region that evidence diene through-bond orbital interaction, whereas absorption peaks are identified in the norbornadiene and norbornadienone spectra that indicate effective through-space orbital interactions. The molecular structure of the cis-cis-[4,4,2]propella-3,8-diene-11,12-dione isomer is such that the indirect through-bond or through-space diene orbital interactions are too weak to be assigned by its C1s NEXAFS spectrum.     

Glass transition of atactic polystyrene probed at the submolecular level by dynamic infrared linear dichroism (DIRLD) spectroscopy

Dynamic infrared linear dichroism (DIRLD) spectroscopy is a rheo-optical characterization technique developed specifically to probe the submolecular dynamics of polymer segments. The technique combines the measurement of submolecular orientation based on the directionally selective absorption of polarized IR light with a small-amplitude oscillatory tensile deformation used in dynamic mechanical analysis. A DIRLD spectroscopic study of atactic polystyrene reveals that a dramatic change in the reorientation behavior of aromatic side groups is observed around the glass transition temperature of 100 °C. The transition point for the main chain backbone, on the other hand, is observed at a much higher temperature around 125 °C. Thus, the macroscopically observable glass transition of polystyrene seems to be dominated by the dynamics of side groups rather than that of the coordinated motions of polymer segments along the backbone. This result suggests a fundamental similarity between the glass transition phenomena of polymers and those of small-molecule inorganic glasses.     

An investigation of the effect of adamantanecarboxylic acids on the structure of epoxy polymers by means of X-ray structural and thermomechanical analyses

The effects of structural modification of epoxy polymers (EPs) with mono- and dicarboxylic adamantane acids are studied. Within a cluster model of the structure of amorphous polymers, the components of the X-ray amorphous halo of the objects under study are identified based on the assumption of the existence of an open-packed matrix and regions of local order. It is established that the introduction of bulk adamantane fragments increases the nonuniformity of the EP structure, which causes a change in molecular and topological characteristics.     

Analysis of the small-angle X-ray scattering of linear polyethylene by a paracrystalline lattice model

Three paracrystalline lattice models for the interpretation of the small angle scattering of polyethylene are discussed: The “lattice model”, the “stapel model” (often referred to as the lamellar stack model) and the “proportional model”. While the applicability of the first model is restricted, the latter models differ in the statistical assumptions of lamellar and interlamellar thickness distributions. The principal advantage of the proportional model over the stapel model is its applicability through the adjustment of only three parameters: long period, crystallinity and one statistical parameter. Small angle X-ray curves of linear polyethylene are interpreted by the proportional model. The results are in good agreement with stapel model calculations.     

Origin of the chemical shift in X‐ray absorption near‐edge spectroscopy at the Mn K‐edge in manganese oxide compounds

The absorption edge in Mn K‐edge X‐ray absorption spectra of manganese oxide compounds shows a shift of several electronvolts in going from MnO through LaMnO₃ to CaMnO₃. On the other hand, in X‐ray photoelectron spectra much smaller shifts are observed. To identify the mechanisms that cause the observed chemical shifts, 1s ionization as well as 1s → “4p” transition energies have been determined by electronic structure calculations on embedded Mn ions and embedded MnO₆ clusters. Systematic variation of the cluster geometry and the cluster embedding showed that the chemical shifts are predominantly determined by two effects: the changes in the Mn 3d occupation and the changes in the Madelung potential. The large chemical shift in the 1s → 4p transition energies between different materials occurs because the two effects do not compensate each other. The chemical shifts obtained for the embedded MnO₆ clusters agree reasonably with the experimental shifts. The small sensitivity to the material observed for the Mn 1s ionization energies is explained by the near cancellation of the effects of the Madelung potential and the 3d occupation of the Mn ion. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Determination of molecular ordering at a buried interface and the effect of interfacial ordering on thin film crystallization by second harmonic generation

It is demonstrated that by using optical second harmonic generation the orientation and alignment of molecules in the interfacial layer between two solids, a thin solid molecular film and a metal substrate, can be determined. The pyridine molecules in the interfacial layer underneath the film are found to align along the [110] direction of the Ag(110) surface with a small tilt angle (approximately 11 degrees) from the surface normal. This interfacial ordering is found to have a notable effect in inducing crystallization at the heterogeneous boundary of the amorphous molecular film.     

Determination of nitrite and bromate in disinfected water samples at the low μg/L level by means of a high-capacity anion exchanger

Bromate and nitrite are formed during the disinfection of water. The German threshold value for nitrite in drinking water is 100 μg/L, for bromate a limit below 50 μg/L has to be established. A self-made high-capacity resin with 2-(dimethylamino)-ethanol functionality in combination with a UV-transparent elution system based on perchloric acid allows the separation and sensitive photometric detection of nitrite and bromate, even if high chloride concentrations are present. The high capacity of the strongly basic resin allows sample volumes of up to 400 μL without disturbance of the separation. Detection limits for bromate and nitrite are 15 and 10 μg/L, respectively. The direct analysis of water samples with high salt contents, such as disinfected pool water, is possible by insertion of an on-line chloride removal step. Received: 30 September 1996 / Revised: 20 December 1996 / Accepted: 28 December 1996     

Completeness of Gaussian orbital and geminal basis sets for linear molecules in L2 and in the first and second Sobolev spaces

Completeness theorems for Gaussian orbital and geminal basis sets of axial symmetry are proved in the space L² of square integrable functions and in the first and second Sobolev spaces H¹ and H². © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 357–384, 1998     

Intramolecular reactions in vinyl polymers as a means of investigation of the propagation step

The Flory-Wall expressions for the fraction of substituents remaining of a vinyl copolymer, when substituents are removed of random from adjacent 1.3 positions, are extended to the case of nonrandam copolymerization. A copolymer composition equation is desired for the case in which the type of monomer preceding the active free radical chain and affects the propagation reaction. The distribution of lengths of sequences in copolymers is discussed.     

IrO2-based disperse-phase electrocatalysts: a complementary study by means of the cavity-microelectrode and ex-situ X-ray absorption spectroscopy

In this work, IrO(2)-based powders are screened by cyclic voltammetry for the determination of the electrochemical active sites and for the qualitative evaluation of the iridium atoms speciation. All results are obtained using a cavity-microelectrode as powder holder, thus exploiting the features of this innovative tool, whose best potentialities have been recently introduced by our group. All the studied materials have been prepared by the sol-gel technique and differ in calcination temperature and method of mixing the metal oxide precursors. The electrochemical results are complemented with the information obtained by X-ray absorption spectroscopy (XAS), that give insights on the local structure of each selected sample, confirming the trends found by cyclic voltammetry and give new and unexpected insights on the powder structural features.     

Electronic structure of cluster assembled nanostructured TiO(2) by resonant photoemission at the Ti L(2,3) edge

The electronic structure of cluster assembled nanostructured TiO(2) thin films has been investigated by resonant photoemission experiments with photon energies across the Ti L(2,3) edge. The samples were produced by supersonic cluster beam deposition with a pulsed microplasma cluster source. The valence band shows resonance enhancements in the binding energy region between 4 and 8 eV, populated by O 2p and hybridized Ti 3d states, and in the region about 1 eV below the Fermi level associated with defects related Ti 3d states. The data show that in as-deposited films Ti atoms are mainly fully (sixfolds) coordinated to oxygen atoms in octahedral symmetry and only a small fraction is in a broken symmetry environment. Since resonant photoemission is closely linked to the local electronic and structural configurations around the Ti atom, it is possible to correlate the resonant photoemission intensity and lineshape with the presence of defects of the films and with the degree of hybridization between the titanium and oxygen atoms.     

Temperature effect on the chemical composition of a polytetrafluoroethylene surface under the irradiation of synchrotron radiation by means of the X-ray photoelectron spectroscopy

The chemical composition and components of a polytetrafluoroethylene (PTFE) surface was investigated as a function of the temperature under the irradiation of synchrotron radiation (SR) by the X-ray photoelectron spectroscopy (XPS). When the temperature of PTFE under the SR irradiation was less than 100 °C, the C-rich surface appeared. With increasing the temperature more than 150 °C, the relative intensity of the F 1s peak to the C 1s peak increased markedly. At the temperatures of 150–180 °C, the C–C component became small and the CF₂ component was dominant. With further increasing the temperature more than 200 °C, CF₃, CF and C–CF components grew in addition to CF₂ component. Based on these XPS results, the temperature effect on the chemical composition and components is discussed.