Febantel: looking for new polymorphs

The aim of this work was to search for new polymorphic forms of febantel, an anthelminthic drug of great present interest for the veterinary industry. Solvent-based recrystallization, thermal and mechanical treatments and spray drying were chosen to discover new solid forms. The solids obtained were physicochemically characterized by thermal techniques (DSC and TG), FTIR spectroscopy, laboratory and synchrotron X-ray powder diffraction and scanning electron microscopy. Our work leads to obtain a new solid form never described in the literature. In particular, the new polymorph was obtained by the anti-solvent method and the crystallization from isopropanol. The experimental conditions of crystallization favorable to the formation of the highest amount of the new solid phase were selected. The new phase shows a thermal, spectroscopic and diffractometric behavior unique. Furthermore, the preliminary structure investigation suggests two possible crystal systems: an orthorhombic or a monoclinic one, with really comparable lattice parameters and cell volume. Measurements put into evidence that the new phase is a metastable polymorph that is in monotropic relationship with the stable and known form.

     

Long-term durability study of perfluoropolymer membranes in low humidification conditions

This paper reports a study on the understanding of the performance decrease mechanisms of polymer electrolyte fuel cells under critical operating conditions. In order to investigate the durability of perfluorosulfonate membranes at low humidification conditions, long-term fuel cell tests have been carried out. Results evidenced a strong effect of low relative humidity on the commercial polymer membrane lifetime. Prolonged dehydration of the membranes led to a decrease of the three-dimensional reaction zone due to the ionomer degradation in the catalytic layer and a continuous loss of material in the membrane evidenced by a thickness decrease. The last effect provoked a collapse of the electrode structure.     

Optimal capping layer thickness for stacked quantum dots

We study the effect of strain on the vertical and lateral self-organization of nanoscale patterns and stacked quantum dots during epitaxial growth. The computational approach is based on the level set method in combination with an atomistic strain code. Strain changes the energetics of microscopic parameters during growth, and thus determines the nucleation sites and the growth of islands and dots. Our results show that strain can lead to vertical alignment as well as lateral organization. Moreover, our simulations suggest that there is an optimal thickness of the capping layer to get the best alignment and most uniform size distribution of stacked quantum dots, and that its variation can be used to control the formation of interesting structures.     

Ueber den Einfluß der Temperatur auf den Brechungsindex von Eiweiß

Ohne Zusammenfassung     

Ueber kolloides Chlorophyll und über einige kolloide Chlorophyllderivate

Ohne Zusammenfassung     

Geometry of singularities for the steady Boussinesq equations

Analysis and computations are presented for singularities in the solution of the steady Boussinesq equations for two-dimensional, stratified flow. The results show that for codimension 1 singularities, there are two generic singularity types for general solutions, and only one generic singularity type if there is a certain symmetry present. The analysis depends on a special choice of coordinates, which greatly simplifies the equations, showing that the type is exactly that of one dimensional Legendrian singularities, generalized so that the velocity can be infinite at the singularity. The solution is viewed as a surface in an appropriate compactified jet space. Smoothness of the solution surface is proved using the Cauchy-Kowalewski Theorem, which also shows that these singularity types are realizable. Numerical results from a special, highly accurate numerical method demonstrate the validity of this geometric analysis. A new analysis of general Legendrian singularities with blowup, i.e., at which the derivative may be infinite, is also presented, using projective coordinates.Research supported in part by the ARPA under URI grant number #N00014092-J-1890.Research supported in part by the NSF under grant number #DMS93-02013.Research supported in part by the NSF under grant #DMS-9306488.     

Ultraviolet photoelectron spectroscopic study of MS σ bonding energy and of spiroconjugation effects in group IVA thiospiranes

He(I) and He(II) ultraviolet photoelectron spectra of tetrathiometallaspiranes

(M = C, Si, Ge, Sn) yield data for quantitative characterization of several structural effects. Spiroconjugation of the lone pair orbitals of the sulfur atoms is indicated by the splitting patterns of the n_s ionization bands in the low IE region (8–9_eV); quantitative evaluation confirms that there is a marked decrease of the spiroconjugation effects with increasing size of M. Energy values are assigned by UP spectral analysis to MS σ bond orbitals which show a small variation with M between ca. 10 and 11 eV, consistent with the small electronegativity variation, and to CS bond orbitals in the spirane rings, fairly constant around 13.5 eV. Ionizations of the quasi-valence 4d orbitals in the Sn derivative are identified at 34.38 and 35.42 eV, and suggest, by comparison with other known Sn compounds, a considerably high overall electronegativity of the ligands in the present compound.     

Evidence for a compensation effect during the temperature-programmed reduction of copper oxide in hydrogen

The reduction of copper oxide derived from basic Cu-carbonate in hydrogen has been studied under temperature-programmed conditions (TPR) and the TPR patterns were analyzed by means of Arrhenius plots at constant conversion (Friedman plots). These plots indicate that the reduction process cannot be described on the basis of constant kinetic parameters and reveal the presence of isokinetic temperatures. These suggest the presence of a compensation effect requiring a modification of the rate equation.