Study of some two-nucleon pick-up reactions in the 1p shell with 39.8 MeV protons

Differential cross sections at a proton energy of 39.8 MeV (lab) have been measured for the following reactions (energies in MeV): ¹²C(p,τ)¹⁰B(g.s., 0.72, 1.74, 2.15, 3.59), ¹⁴C(p, t)¹²C(g.s., 4.43), ¹⁴N(p, τ)¹²C(g.s., 4.43), and ¹⁶O(p,τ)¹⁴N(g.s., 2.31, 3.95). A zero-range DWBA analysis of the data has been performed using the 1p shell wave functions of Cohen and Kurath. The fits we find are overall somewhat worse in shape than those found in the (p, t) survey of the 1p shell performed by Kahana and Kurath, the principal reason being that of the (p, τ) transitions which proceed with both L = 0 and L = 2 components we find several which occur with a much weaker L = 0 strength than the calculations predict. When ratios of experimental integrated cross sections to DWBA integrated cross sections are compared for all transitions, an rms deviation about the mean of 39% of the mean is found, whereas if only ratios for transitions from a given target nucleus are compared, then the rms deviations are considerably smaller.     

Low temperature rearrangement of amine cured epoxy resins

Studies are reported on the rearrangements which occur in epoxy resins when heated above 497 K. The nature of the processes which occur in the resin and in model compounds is discussed with reference to changes in the ultraviolet, infra-red and ¹³C and ¹H NMR spectra. Chain scission is preceded by dehydration of the glycidyl segment and occurs at the CN and OC bonds, generating phenoxy radicals in the latter case. The observed coloration of the resin at high temperatures is associated with the formation of a polyenyl structure and the possible generation of quinoid and cyclic conjugated nitrogen-containing aromatic ring structures.     

Constitutive modeling of radiation effects on the permanent set in a silicone elastomer

When a networked polymeric composite under high stress is subjected to irradiation, the resulting chemical changes like chain scissioning and crosslink formation can lead to permanent set and altered elastic modulus. Using a commercial silicone elastomer as a specific example we show that a simple 2-stage Tobolsky model in conjunction with Fricker's stress transfer function can quantitatively reproduce all experimental data as a function of radiation dosage and the static strain at which radiation is turned on, including permanent set, stress-strain response, and net crosslink density.     

Electrochemical Modification of Polypeptides at Selenocysteine

Mild strategies for the selective modification of peptides and proteins are in demand for applications in therapeutic peptide and protein discovery, and in the study of fundamental biomolecular processes. Herein, we describe the development of an electrochemical selenoetherification (e-SE) platform for the efficient site-selective functionalization of polypeptides. This methodology utilizes the unique reactivity of the 21st amino acid, selenocysteine, to effect formation of valuable bioconjugates through stable selenoether linkages under mild electrochemical conditions. The power of e-SE is highlighted through late-stage C-terminal modification of the FDA-approved cancer drug leuprolide and assembly of a library of anti-HER2 affibody conjugates bearing complex cargoes. Following assembly by e-SE, the utility of functionalized affibodies for in vitro imaging and targeting of HER2 positive breast and lung cancer cell lines is also demonstrated.     

Toward multistation rotaxanes using metalloporphyrin coordination templating

This paper describes some approaches toward the templated synthesis of rotaxanes incorporating strapped metalloporphyrin moieties as the shuttle unit, with the thread component containing both a neutral diimide "station" and a functionalized pyridine moiety, the latter acting not only as a template but also as a second binding motif. In the first instance, the use of appropriately 3,5-difunctionalized pyridine esters and naphthoquinol-strapped rhodium(III) chloride porphyrins in a stoppering approach to rotaxanes produced only unlinked components: the flexibility of the strap allowed sufficient room for the potential thread unit to bind on the same face of the porphyrin as the strap, while not being interlocked through it. An alternative strategy involving a 1,3-dipolar cycloaddition reaction (a "click" reaction) between azides and alkynes, producing triazole linkers in the thread component of rotaxanes, was more successful. Both porphyrinic (zinc, free base, and rhodium(III) derivatives) and crown ether rotaxanes were successfully produced, with multifunctional (triazole and naphthodiimide) thread units. The potential for molecular motion through the use of stimuli such as acid, solvent, and competing ligands was investigated, with limited success. The same cycloaddition methodology was extended to pyridine-templated analogues of the thread components in the Rh(III)-strapped porphyrins, but again, only unlinked thread and porphyrin shuttle units were produced.     

Pyromellitamide aggregates and their response to anion stimuli

The N,N',N',N'-1,2,4,5-tetra(ethylhexanoate) pyromellitamide is found to be capable of both intermolecular aggregation and binding to small anions. It is synthesized by aminolysis of pyromellitic anhydride with ethanolamine, followed by a reaction with hexanoyl chloride. The single-crystal X-ray structure of the pyromellitamide shows that it forms one-dimensional columnar stacks through an intermolecular hydrogen-bonding network. It also forms self-assembled gels in nonpolar solvents, presumably by a hydrogen-bonding network similar to the solid-state structure as shown by IR and XRD studies. Aggregation by intermolecular hydrogen bonding of the pyromellitamide is also observed by NMR and IR in solution. Fitting of NMR dilution data for pyromellitamide in d6-acetone to a cooperative aggregation model gave KE=232 M-1 and positive cooperativity of aggregation (rho=0.22). The pyromellitamide binds to a range of small anions with the binding strength decreasing in the order chloride>acetate>bromide>nitrate approximately iodide. The data indicate that the pyromellitamide binds two anions and that it displays negative cooperativity. The intermolecular aggregation of the pyromellitamide can also be altered using small anion stimuli; anion addition to preformed self-assembled pyromellitamide gels causes their collapse. The kinetics of anion-induced gel collapse are qualitatively correlated to the binding affinities of the same anions in solution. The cooperative anion binding properties and the sensitivity of the self-assembled gels formed by pyromellitamide toward anions could be useful in the development of sensors and switching/releasing devices.     

Comparison of amorphous iridium water-oxidation electrocatalysts prepared from soluble precursors

Electrodeposition of iridium oxide layers from soluble precursors provides a route to active thin-layer electrocatalysts for use on water-oxidizing anodes. Certain organometallic half-sandwich aqua complexes of iridium form stable and highly active oxide films upon electrochemical oxidation in aqueous solution. The catalyst films appear as blue layers on the anode when sufficiently thick, and most closely resemble hydrous iridium(III,IV) oxide by voltammetry. The deposition rate and cyclic voltammetric response of the electrodeposited material depend on whether the precursor complex contains a pentamethylcyclopentadieneyl (Cp*) or cyclopentadienyl ligand (Cp), and do not match, in either case, iridium oxide anodes prepared from non-organometallic precursors. Here, we survey our organometallic precursors, iridium hydroxide, and pre-formed iridium oxide nanoparticles. From electrochemical quartz crystal nanobalance (EQCN) studies, we find differences in the rate of electrodeposition of catalyst layers from the two half-sandwich precursors; however, the resulting layers operate as water-oxidizing anodes with indistinguishable overpotentials and H/D isotope effects. Furthermore, using the mass data collected by EQCN and not otherwise available, we show that the electrodeposited materials are excellent catalysts for the water-oxidation reaction, showing maximum turnover frequencies greater than 0.5 mol O(2) (mol iridium)(-1) s(-1) and quantitative conversion of current to product dioxygen. Importantly, these anodes maintain their high activity and robustness at very low iridium loadings. Our organometallic precursors contrast with pre-formed iridium oxide nanoparticles, which form an unstable electrodeposited material that is not stably adherent to the anode surface at even moderately oxidizing potentials.     

Mechanical properties of polymer mixtures: Effect of compatibility

The effect of compatibility on the mechanical properties of polymer mixtures is discussed. From solution thermodynamics we conclude that for compatible mixtures of high molecular weight polymers the excess internal energy of mixing must be negative. For most polymer mixtures, this results in a negative excess volume of mixing. The excess entropy of mixing is therefore also negative. These general conclusions point to a stiffening of the solid lattice and a reduction in chain mobility. In dynamic mechanical behavior, the relaxation of molecular moieties responsible for the specific interaction should be shifted to intermediate temperatures, and the relaxation of other moieties should be shifted to higher temperatures if the potential well is inter-molecular. In finite deformation behavior craze initiation stress and shear yielding stress should both be higher than a linear combination of the properties of the pure components.