Aggregation of sulfonated poly(p-phenylene terephthalamide) in dilute solutions

The viscosities of dilute solutions of poly(p-phenylene terephthalamide), PPTS, in dimethylacetamide, water, and their mixtures were determined. The reduced viscosity plot in dimethylacetamide shows a negative slope. When the water content in the mixed solvent in 90% or higher, there is an upswing in the reduced viscosity values at concentrations below 0.1 g/dL. The latter behavior suggests a “polyelectrolyte” effect. However, an association model was found to be able to explain the viscosity behaviors in both solvents. ©1995 John Wiley & Sons, Inc.     

Tunable dielectric thin films by aqueous, inorganic solution-based processing

Sub-micron, nanolaminated, dielectric thin films comprised of amorphous aluminum oxide phosphate (AlPO) and hafnium oxide sulfate (HafSOx) layers were fabricated in open-air conditions from aqueous inorganic precursors by spin coating with minimal thermal processing. These nanolaminated thin film insulators display an averaging effect of effective dielectric permittivity in devices with controlled AlPO:HafSOx thickness ratios, enabling tunable dielectric properties. X-ray reflectivity measurements were used to characterize film thickness, smoothness, and uniformity. Scanning electron microscopy was used to analyze final nanolaminated devices. Electrical characterization of metal-insulator-insulator-metal capacitors revealed tunable relative dielectric constants ranging from approximately 5–10 with loss tangents less than 2% at 10 kHz in devices with approximately 300 nm total dielectric thickness. The results suggest a simple, inexpensive processing approach for fabricating devices that require insulating layers with specific dielectric properties.     

Theoretical studies on cerium nickel aluminides: polar intermetallics with heavy fermion behavior

The electronic structures of Ce₄Ni₆Al₂₃, CeNiAl₄, CeNi₂Al₅, CeNiAl and CeNi₄Al have been calculated using the TB-LMTO-ASA (tight-binding, linear muffin-tin orbital, atomic-spheres approximation) approach to probe relationships between chemical bonding and physical properties in this series of intermetallic compounds. Analysis from crystal orbital Hamilton populations (COHP) reveal that the Al-rich compounds may be considered as “polar intermetallic” because the Fermi level coincides to the separation of bonding and antibonding states of the Ni-Al framework. On the other hand, although the densities of states (DOS) of CeNiAl suggest “polar intermetallic” behavior, the bonding is more complex. Finally, the Ni-rich example, CeNi₄Al, has significant Ni-3d character at the Fermi level. The results of these calculations are also discussed in connection with heavy fermion or possible valence fluctuation behavior observed for some of these intermetallic compounds: those showing exceptional properties also exhibit significant “lattice covalency” between Ce and the Ni-Al nets.     

Phosphine triggered [3+2] allenoate-acrylate annulation: a mechanistic enlightenment

A mechanistic study of phosphine-mediated [3+2] annulation of allenoate and acrylate is presented. The insight gained has identified that (1) [3+2] cycloaddition proceeds through a stepwise mechanism and (2) the involvement of a molecule of water, which services as a proton-shuttle, is essential for annulated product formation.     

The reaction of fluoroxysulfate,sof-. liith aromatic compounds: catalysis by acids

The effects of the acid catalysts HF, H₂S0₄, BF₃, CF₃S0₃H, FSO₃H, and SbF₅.FS0₃H on the reactions of CsSO₄F with toluene, nitrobenzene and naphthalene in CH₃CN have been studied. The catalys general accelerate reaction and enhance yields of ring-fluorinated products. Efficacity roughly parallels H₀ of the acids, though H₂S0₄ is more effective than stronger acids for the fluorination of naphthalene. Combined room temperature yields of fluorine-substituted products are as much as 45-50% for toluene and naphthalene and up to 30% for nitrobenzene. The predominant products are o-fluorotoluene, 1- fluoronaphthalene, and m-fluoronitrobenzene. With naphthalene the isomer selectivity is considerably less than in the absence of catalyst. Low yields (10%) of monomeric oxygenated products are obtained, along with considerable oxygen- and fluorine-containing dimers and higher polymers. The results are interpreted in terms of acid-catalyzed electrophilic fluorination or oxygenation, followed by further fluorination and/or free-radical-induced oxidative coupling of the oxygenated products.     

Adsorption of beta-hairpin peptides on the surface of water: a neutron reflection study

Neutron reflectivity has been used to determine the thickness and surface coverage of monolayers of two 14-residue beta-hairpin peptides adsorbed at the air/water interface. The peptides differed only in that one was labeled with a fluorophore, while the other was not. The neutron reflection measurements were mainly made in null reflecting water, NRW, containing 8.1% D(2)O. Under this isotopic contrast the water is invisible to neutrons and the specular signal was then only from the peptide layer. At the highest concentration of ca. 4 microg/mL studied, the area per peptide molecule (A) was found to be 230 +/- 10 and 210 +/- 10 A(2) for the peptides with and without a BODIPY-based fluorophore, respectively. The thickness of the peptide layers was about 10 A for a Gaussian distribution. With decreasing bulk peptide concentration, both surface excess and layer thickness showed a steady trend of decrease. While the neutron results clearly indicate structural changes within the peptide monolayers with increasing bulk concentration, the outstanding structural feature is the formation of rather uniform peptide layers, consistent with the structural characteristics typical of beta-strand peptide conformations. These structural features are well supported by the parallel measurements of the adsorbed layers in D(2)O. With this isotopic contrast the neutron reflectivity provides an estimate about the extent of immersion of the peptide layers into water. The results strongly suggest that the 14-mer peptide monolayers were fully afloat on the surface of water, with only the carboxy groups on Glu residues hydrated.     

Quantum mechanical tunneling in synaptic and ephaptic transmission

Quantum mechanical tunneling theory is applied to the problem of synaptic vesicle release and to the problem of electric transmission at the ephaptic junction. In the synapse the tunneling produces conformational changes in macromolecules to open and close vesicle macrogates. Quantum mechanical tunneling as a basis for charge transfer and physical release of vesicles at junction membranes provides a unified concept of ephaptic and synaptic transmission. Details of this model are in agreement with experimental data for miniature endplate potential frequency and delay effects as a function of polarization, osmotic pressure, and temperature. The theory accounts for anatomical specializations at the synaptic cleft and the narrow junction observed for the ephapse.     

Catalytically active mu-Oxodiiron(IV) oxidants from Iron(III) and dioxygen

The reaction between an Fe(III) complex and O(2) to afford a stable catalytically active diiron(IV)-mu-oxo compound is described. Phosphonium salts of orange five-coordinated Fe(III)-TAML complexes with an axial aqua ligand ([PPh(4)]1-H(2)O, tetraamidato macrocyclic Fe(III) species derived from 3,3,6,6,9,9-hexamethyl-3,4,8,9-tetrahydro-1H-1,4,8,11-benzotetraazacyclotridecine-2,5,7,10(6H,11H)-tetraone) react rapidly with O(2) in CH(2)Cl(2) or other weakly coordinating solvents to produce black mu-oxo-bridged diiron(IV) complexes, 2, in high yields. Complexes 2 have been characterized by X-ray crystallography (2 cases), microanalytical data, mass spectrometry, UV/Vis, Mossbauer, and (1)H NMR spectroscopies. Mossbauer data show that the diamagnetic Fe-O-Fe unit contains antiferromagnetically coupled S = 1 Fe(IV) sites; diamagnetic (1)H NMR spectra are observed. The oxidation of PPh(3) to OPPh(3) by 2 was confirmed by UV/Vis and GC-MS. Labeling experiments with (18)O(2) and H(2)(18)O established that the bridging oxygen atom of 2 derives from O(2). Complexes 2 catalyze the selective oxidation of benzylic alcohols into the corresponding aldehydes and bleach rapidly organic dyes, such as Orange II in MeCN-H(2)O mixtures; reactivity evidence suggests that free radical autoxidation is not involved. This work highlights a promising development for the advancement of green oxidation technology, as O(2) is an abundant, clean, and inexpensive oxidizing agent.