Silica Dissolution-Redeposition in Gels and Aerogels

Silica dissolution-redeposition phenomenon is investigated on a microscopic scale using Small Angle Xray Scattering (SAXS) method. The changes occurring in the microtexture are different according to the procedure used to dry the gels. A xerogel, a CO₂ supercritically dried aerogel and an alcohol supercritically dried aerogel were compared. Silica does not dissolve in CO₂ supercritical. The transformation of heat treated gels into CO₂ supercritically dried aerogels is demonstrated as having minor effect on the microtexture. SAXS data show oscillations around Porod law I(Q) Q ^–4 where Q is the wave vector. The oscillations are more pronounced for alcohol dried aerogel than for xerogel or CO₂ dried aerogel. These oscillations are related to curvatures of the surface i.e. the surface roughness. It was found that according to silica dissolution extent, the solid network surface was continuously smoothened in alcohol supercritically dried aerogel. The average chord length of the solid phase is shifted to larger values while the distribution seems to be more narrow. This effect does not give rise to the formation of closed pores.     

Solid-state studies on crystalline and glassy flutamide

Flutamide usually crystallizes in the orthorhombic non-centrosymmetric space group Pna2₁ (from I) and melts atT _fus =384 K with Δ_fus H=30 kJ·mol^?1. It may be obtained in the glassy state (T _g =272 K) by quenching the melt. Although evidence of polymorphism could not be obtained by means of crystallography, DSC studies of the recrystallization process indicate that a metastable form (form II) occurs first and is transformed into the stable form at room temperature. ΔH for the transition I→II (2.52 kJ·mol^?1) is close to the difference in energy (about 2 kJ·mol^?1) calculated for the two possible conformers of flutamide.     

Initiation and propagation steps in the equibinary polymerization of butadiene by bis(h3-allylnickel trifluoroacetate)

Polymerization of butadiene by bis(h³-allylnickel trifluoroacetate) in benzene and o-dichlorobenzene solvents yields an equibinary 1,4-polybutadiene, containing equal amounts of cis and trans isomers. Initiation proceeds by addition of the allylic moiety of the initiator to a butadiene molecule. The rate of initiation is high enough to ensure complete consumption of the catalyst for a monomer/catalyst molar ratio of about 10 at 5°C. The propagation exhibits the characteristics of a “living” polymerization: the molecular weight is proportional to the conversion, and at the end of the reaction, the average degree of polymerization is equal to the monomer/catalyst molar ratio. Living polybutadienyl-nickel trifluoroacetate is able to reinitiate not only butadiene polymerization but also allene polymerization. However, for high [monomer]/[catalyst] ratios, conversion-dependent transfer reactions limit the molecular weight to 7000 in benzene and to 70,000 in bulk polymerization in the presence of small amounts of o-dichlorobenzene.     

Dual fluorescence of 4-N,N-dimethylaminopyridine. Role of hydrogen-bonded complex in the ground state

A correlation is shown between the appearance of the dual fluorescence of 4-N,N-dimethylaminopyridine (DMAP) solutions and the formation of hydrogen-bonded of complexes in the ground state. A comparative absorption study between pyridine, N,N-dimethylaniline and DMAP shows that the hydrogen-bonded complex is situated on the amino nitrogen of DMAP. A “pretwisted” conformation of DMAP in the ground state isassumed due to this hydrogen-bonded complex. Simulations by intermolecular interaction calculations and spectroscopic calculations (CNDO/s) confrim the “twisting” influence of water molecules (and/or any other hydrogen bonding) on the amine in the ground state. This “pretwisting” in the ground state by hydrogen bonding is common in many other aromatic amines. Moreover, the deforming role of hydrogen bonding in the ground state seems to be a general phenomenon in flexible aromatic molecules.     

Untersuchungen an Metallchelaten mit Liganden des Cuproin- und Ferrointyps VI Untersuchungen zur Struktur und Bindung in Sulfato-(cuproin)- und Sulfato-(ferroin)- kupfer(II)-Chelaten

Studies on Metal Chelates with Cuproine and Ferroine Type Ligands. VI. Investigations on Structure and Bonding in Sulphato(cuproine) and Sulphato-(ferroine)copper(II) Chelates Copper(II) chelates of the general formula Cu(N–N)SO₄, where (N–N) = bipy, phen, bich, dmch, and dmp², were examined by means of the IR-, VIS-, and ESR-techniques. The structures derived from these results are given. For CubipySO₄ · 2 H₂O and CuphenSO₄ · 2 H₂O the MO-parameters were calculated. Strong covalent out-of-plane-π-bondings indicating π-acceptor properties of the ligands were found to be present.     

Block copolymerization of 3,3-dimethyl-2-oxetanone. II. Synthesis of polydimethylsiloxane-containing copolymers

Carboxy-terminated polydimethylsiloxanes (PDMSs) have been prepared by anchoring the functional end group on the main chain via a hydrolytically stable Si? C bond. The neutralized carboxy-terminated PDMSs are effective initiators of the 3,3-dimethyl-2-oxetanone (or pivalolactone) block copolymerization. The yield of block copolymers depends on the initiation efficiency of the carboxylate end groups, but this limit can be overcome by the use of more than one carboxylate group per end chain, and “palm-tree” structures are obtained. The ability to crystallize of poly(3,3-dimethyl-2-oxetanone) is significantly depressed when this polyester is engaged in PDMS-rich block copolymers, but the observed heterophase morphologies (350 Å) are uniformly fine.     

Identification of noise in linear data sets by factor analysis

With the use of atomic and nuclear methods to analyze samples for a multitude of elements, very large data sets have been generated. Due to the ease of obtaining these results with computerized systems, the elemental data acquired are not always as thoroughly checked as they should be leading to some, if not many, bad data points. It is advantageous to have some feeling for the trouble spots in a data, set before it is used for further studies. A technique which has the ability to identify bad data points, after the data has been generated, is classical factor analysis. The ability of classical factor analysis to identify two different types of data errors make it ideally suited for scanning large data sets. Since the results, yielded by factor analysis indicate correlations between parameters, one must know something about the nature of the data set and the analytical techniques used to obtain it to confidentially isolate errors.     

Closed and open-shell atomic oxygen on silver: two distinct patterns of the O<Subscript>1s</Subscript> binding energy and X-ray absorption O <Emphasis Type="Italic">K</Emphasis>-edge spectra as revealed by density functional theory

The electronic structure of atomic oxygen adsorbed species is studied by means of the density functional theory in the context of the ethylene epoxidation on the silver surface. The adsorbed oxygen species are modeled by the Ag₂O molecule either in its closed (¹A₁) or open-shell states (³B₁ and ¹B₁). In both open-shell states the 1s level appears to be lower than that in ¹A₁ by about 2 eV. This is apparently a sequence of the separation of electron pair, occupying the *-type highest occupied molecular orbital (HOMO), decreasing the electron density at the oxygen center. Such variation of the O_1s level for closed and open-shell Ag₂O states seems to explain the X-ray photoelectron spectroscopy (XPS) data concerning two distinct atomic oxygen species on silver surface having the O_1s binding energy of about 528 and 530 eV, called nucleophilic and electrophilic oxygen, respectively. The X-ray absorption O K-edge spectra (XANES) calculated for two types of the Ag₂O states by means of multiple-scattered-X-based approach appears to be in a qualitative agreement with those experimentally recorded for nucleophilic and electrophilic oxygen.     

Nitrosyl-Metallkoordinationsverbindungen. VI. Synthese und Eigenschaften von Cobalt- und Eisenkomplexen des Typs M(S,S)(P,P)NO

Nitrosyl Metal Coordination Compounds. VI. Syntheses and Properties of Cobalt and Iron Complexes of the Type M(S,S)(P,P)NO Penta-coordinated neutral complexes of the composition M(S,S)(P,P)NO, with M = Co, Fe; S,S = malonitrildithiolate (mnt), P,P = tetraphenylethylenediphosphine (diphos), and triphenylphosphine (PPh₃) have been synthesized and investigated by means of molecular weights measurement, IR, UV/VIS, and ESCA spectroscopy as well as magnetic measurements. Square pyramidale and trigonal bipyramidale structures are discussed.