Photo-oxidation of polymers: Part IV—Influence of the processing conditions on the photo-oxidative stability of polystyrene

Processing of polystyrene at 160°C for different periods in the closed chamber of a Brabender Plastograph results in no appreciable oxidation. Therefore, the photo-oxidative stability of the polymer, as measured from retention of the ultimate tensile strength, is not affected by such treatment. On the other hand, acetophenone groups, the concentration of which increases with processing time, are produced at 160°C in the open chamber of the Plastograph as a consequence of extensive thermal oxidation. A quantitative correlation is found between the rate of change of the ultimate tensile strength of polystyrene and the extent of thermal oxidation. It is also demonstrated that a phenolic antioxidant, although it efficiently inhibits the thermal oxidation, has no specific effect on the photo-oxidation of polystyrene. A hindered amine light stabiliser, however, exhibits a pronounced photo-protective effect. A mixture of these additives combines the benefits of both.     

Parametrization of the deuteron wave function of the Paris NN potential

We present a convenient analytical parametrization, in both configuration and momentum spaces, of the deuteron wave-function calculated with the Paris potential.     

On the role of energy separated in fission process,excitation energy and reaction channels effects in the isomeric ratios of fission product <Superscript>135</Superscript>Xe in photofission of actinide elements

In this work we present the isomeric ratio of fission product ¹³⁵Xe in the photo-fission of actinide elements ²³²Th, ²³³U and ²³⁷Np induced by end-point bremsstrahlung energies of 13.5, 23.5 and 25.0 MeV which were determined by the method of inert gaseous flow. The data were analyzed, discussed and compared with the similar data from literature to examine the role of energy separated in fission process, excitation energy and reaction channels effects.     

Experimental and theoretical study of the reaction of the ethynyl radical with nitrous oxide, C2H + N2O

We investigated the rate constants and reaction mechanism of the gas phase reaction between the ethynyl radical and nitrous oxide (C(2)H + N(2)O) using both experimental methods and electronic structure calculations. A pulsed-laser photolysis/chemiluminescence technique was used to determine the absolute rate coefficient over the temperature range 570 K to 836 K. In this experimental temperature range, the measured temperature dependence of the overall rate constants can be expressed as: k(T) (C(2)H + N(2)O) = 2.93 × 10(-11) exp((-4000 ± 1100) K/T) cm(3) s(-1) (95% statistical confidence). Portions of the C(2)H + N(2)O potential energy surface (PES), containing low-energy pathways, were constructed using the composite G3B3 method. A multi-step reaction route leading to the products HCCO + N(2) is clearly preferred. The high selectivity between product channels favouring N(2) formation occurs very early. The pathway corresponds to the addition of the terminal C atom of C(2)H to the terminal N atom of N(2)O. Refined calculations using the coupled-cluster theory whose electronic energies were extrapolated to the complete basis set limit CCSD(T)/CBS led to an energy barrier of 6.0 kcal mol(-1) for the entrance channel. The overall rate constant was also determined by application of transition-state theory and Rice-Ramsperger-Kassel-Marcus (RRKM) statistical analyses to the PES. The computed rate constants have similar temperature dependence to the experimental values, though were somewhat lower.     

Ultrasmall reduced graphene oxide with high near-infrared absorbance for photothermal therapy

We developed nanosized, reduced graphene oxide (nano-rGO) sheets with high near-infrared (NIR) light absorbance and biocompatibility for potential photothermal therapy. The single-layered nano-rGO sheets were ～20 nm in average lateral dimension, functionalized noncovalently by amphiphilic PEGylated polymer chains to render stability in biological solutions and exhibited 6-fold higher NIR absorption than nonreduced, covalently PEGylated nano-GO. Attaching a targeting peptide bearing the Arg-Gly-Asp (RGD) motif to nano-rGO afforded selective cellular uptake in U87MG cancer cells and highly effective photoablation of cells in vitro. In the absence of any NIR irradiation, nano-rGO exhibited little toxicity in vitro at concentrations well above the doses needed for photothermal heating. This work established nano-rGO as a novel photothermal agent due to its small size, high photothermal efficiency, and low cost as compared to other NIR photothermal agents including gold nanomaterials and carbon nanotubes.     

Solvent-controlled selective synthesis of a trans-configured benzimidazoline-2-ylidene palladium(II) complex and investigations of its Heck-type catalytic activity

Reaction of N,N′-dimethylbenzimidazolyl iodide (A) with Pd(OAc)₂ in DMSO gives selectively trans-bis(N,N′-dimethylbenzimidazoline-2-ylidene) palladium(II) diiodide (trans-2) in 77% yield. The selective formation of the trans-coordination isomer and thus the cis-trans rearrangement is driven by the insolubility of trans-2 in DMSO. X-ray single-crystal diffraction analysis and ¹³C NMR spectroscopy confirm the trans-geometry of the square planar Pd(II) complex. Catalytic studies show that cis-1 and trans-2 are highly efficient in the Mizoroki-Heck coupling reaction of aryl bromides and activated aryl chlorides both in DMF and [N(n-C₄H₉)₄]Br as ionic liquid. The catalytic activities of Pd(II) complexes with N-heterocyclic carbene ligands derived from benzimidazole are comparable to their imidazole-derived analogues.     

Multi C−H Functionalization Reactions of Carbazole Heterocycles via Gold-Catalyzed Carbene Transfer Reactions

Herein we describe a multiple C−H functionalization reaction of carbazole heterocycles with diazoalkanes. We show that gold catalysts play a distinct role in enabling a multiple C−H functionalization reaction to introduce up to six carbene fragments onto molecules containing multiple carbazole units or to link multiple carbazole units into a single molecule. A one-pot stepwise approach enables the introduction of two different carbene fragments to allow orthogonal deprotection and straightforward derivatization.     

Cytotoxic triterpene saponins from the mangrove Aegiceras corniculatum

Using various chromatographic separations, sixteen compounds, including one new triterpene saponin named aegicoroside A (1), were isolated from the leaves of the Vietnamese mangrove Aegiceras corniculatum. Their structures were determined by spectroscopic methods such as 1D and 2D NMR and HR-ESI-MS. The cytotoxic activities of the isolated compounds against MCF7 (breast), HCT116 (colon), B16F10 (melanoma), and A549 (adenocarcinoma) cancer cell lines were also evaluated. Strong cytotoxicity was observed for sakurasosaponin (2) against all four cancer cell lines and for sakurasosaponin methyl ester (3) against MCF7, A549, and HCT116 cell lines with IC₅₀ values ranging from 2.89 ± 0.02 to 9.86 ± 0.21 μM.     

Asymmetric construction of quaternary carbon stereocenters: high stereoselection in Mukaiyama aldol reactions of 2-siloxyindoles with chiral aldehydes

[reaction: see text] A new synthesis of enantiopure 3,3-disubstituted oxindoles by stereoselective Mukaiyama aldol reaction of 3-substituted 2-siloxyindoles and chiral, enantiopure aldehydes having nitrogen or oxygen substituents at the alpha carbon is described. When the C3 substituent of the prochiral nucleophile is aryl or heteroaryl, stereoselectivity is high (10-80:1).