Structural and optical characterization of pyrolytic carbon derived from novolac resin

The structural and optical properties of technologically interesting pyrolytic carbons formed from cured novolac resin and cured novolac/biomass composites were studied by X-Ray Diffraction Analysis (XRD), and Fourier Transform Infrared (FTIR), Raman and Photoluminescence (PL) spectroscopy. Pyrolysis of the cured materials took place at temperatures in the range 400–1000 °C. The most important weight loss, shrinkage and structural changes of pyrolyzed composites are observed at temperatures up to 600 °C due to the olive stone component. In the same temperature range, the changes in pyrolyzed novolac are smaller. The spectroscopic analysis shows that novolac pyrolyzed up to 900 ^°C has less defects and disorder than the composites. However, above 900 ^°C, pyrolyzed novolac becomes more disordered compared to the pyrolyzed composites. It is concluded that partial replacement of novolac by olive stone in the composite materials leads to the formation of a low cost, good quality product.     

On the detection of both carbonyl and hydroxyl oxygens in amino acid derivatives: a O NMR reinvestigation

The hypothesis and the conclusions of previous ¹⁷O NMR studies on the detection of both oxygens of the carboxylic group of Boc-[¹⁷O]Tyr(2,6-diClBzl)-OH in DMSO-d₆ solution (Tetrahedron Lett.2000, 41, 8651) are reconsidered. The appearance of two discrete resonances at 340 and 175 ppm of this protected amino acid is not now attributed: (a) to the reduction of the intramolecular conformational exchange rate, due to the effect of intramolecular hydrogen bonding of the hydroxy part of the carboxyl with the carbonyl oxygen of the Boc-group, and (b) to the effect of solvent viscosity, suggested in the mentioned study. The cause of this phenomenon is now attributed to a strong hydrogen bonding of the polar proton acceptor solvent DMSO with the carboxy group, which effectively reduces the proton exchange rate, thus becoming slow on the ¹⁷O NMR time scale.     

A convenient method for the preparation of primary amines using tritylamine

A simple method for the preparation of primary amines by treating N-tritylamines with trifluoroacetic acid has been established. The N-tritylamines were prepared by the reaction of alkyl halides or alkyl p-toluenesulfonates with tritylamine, or by the reaction of alkyl bromides with lithium tritylamide.     

Structural and catalytic aspects of cutinase in w/o microemulsions

 Structural and catalytic properties of cutinase were studied in bis(2-ethylhexyl) sodium sulfo-succinate (AOT)-isooctane microemulsion systems. The effect of the water content of the microemulsions on the cutinase activity on an esterification reaction of lauric acid with pentanol showed that cutinase followed a bell-shaped profile presenting a maximum at w _o=9, with w _o=[H₂O]/[AOT]. Kinetic studies allowed the determi-nation of the apparent parameters K _m and V _max. Electron paramagnetic resonance (EPR) spectroscopy studies of active site labeled cutinase in microemulsions with varying w _o values showed that in all microemulsions, the mobility of the label is higher than in the aqueous solution. Furthermore, it was found that the maximum of the enzyme activity did not correspond to a reduced active site mobility. Up to w _o=9 there was an increase of both activity and active site mobility. As the water content of the system became higher, the mobility of the bound spin label further increased whereas the enzymatic activity dropped considerably. Received: 20 December 1996 Accepted: 24 February 1997     

The effect of lattice constant on the storage capacity of hydrogen hydrates: a Monte Carlo study

ABSTRACT

We use grand canonical Monte Carlo simulations to examine the effect of the size of hydrate cavities (as reflected through the lattice constant of the hydrate unit cell) on the efficiency of clathrate hydrates in storing hydrogen gas. With this approach, the hydrate lattice is treated as a solid substrate where gas absorption takes place. Of practical interests are cases, where the lattice-size parameters are changed in such a way that they can promote/enhance multiple cavity occupancy, namely the presence of more than one guest-gas molecule in the same hydrate cavity. This phenomenon is commonly observed in the case of hydrogen hydrates and could increase their storage capacity. A parametric analysis is also carried out to quantify the correlation between the change in the lattice constant and the storage capacity since small changes in the size of the crystal unit cell may induce significant changes in the storage capacity especially in cases where multiple cavity occupancy occurs.     

Absolute rate coefficient determination and reaction mechanism investigation for the reaction of Cl atoms with CH2I2 and the oxidation mechanism of CH2I radicals

The gas-phase reaction of atomic chlorine with diiodomethane was studied over the temperature range 273-363 K with the very low-pressure reactor (VLPR) technique. The reaction takes place in a Knudsen reactor at pressures below 3 mTorr, where the steady-state concentration of both reactants and stable products is continuously measured by electron-impact mass spectrometry. The absolute rate coefficient as a function of temperature was given by k = (4.70 +/- 0.65) x 10-11 exp[-(241 +/- 33)/T] cm3molecule-1s-1, in the low-pressure regime. The quoted uncertainties are given at a 95% level of confidence (2sigma) and include systematic errors. The reaction occurs via two pathways: the abstraction of a hydrogen atom leading to HCl and the abstraction of an iodine atom leading to ICl. The HCl yield was measured to be ca. 55 +/- 10%. The results suggest that the reaction proceeds via the intermediate CH2I2-Cl adduct formation, with a I-Cl bond strength of 51.9 +/- 15 kJ mol-1, calculated at the B3P86/aug-cc-pVTZ-PP level of theory. Furthermore, the oxidation reactions of CHI2 and CH2I radicals were studied by introducing an excess of molecular oxygen in the Knudsen reactor. HCHO and HCOOH were the primary oxidation products indicating that the reactions with O2 proceed via the intermediate peroxy radical formation and the subsequent elimination of either IO radical or I atom. HCHO and HCOOH were also detected by FT-IR, as the reaction products of photolytically generated CH2I radicals with O2 in a static cell, which supports the proposed oxidation mechanism. Since the photolysis of CH2I2 is about 3 orders of magnitude faster than its reactive loss by Cl atoms, the title reaction does not constitute an important tropospheric sink for CH2I2.     

Modified π-states in ion-irradiated carbon

CVD polycrystalline diamond film, pulse laser-deposited (PLD) carbon film and highly oriented pirolitical graphite (HOPG) as reference, were modified by means of Ar⁺ ion bombardment and characterized by means of Raman scattering, transmission electron microscopy, electron-diffraction (TEM), reflected electron energy loss specroscopy (REELS) and X-ray photoelectron spectroscopy (XPS) techniques. It was found that the diamond was transferred to a carbon with halo-like morphology and disordered stack of graphene segments. Instead of the well-known electron energy loss peak of graphite at 6.5 eV, a new REELS peak appeared at 4-5 eV energies. The observed effect was explained by the modification of π-system in carbon films as a consequence of the formation of non-planar, nanometer-sized graphitic planes.     

Difficult reptation of star‐shaped polystyrenes in their blends with poly(methyl methacrylate)

Films of polystyrene/poly(methyl methacrylate) (PS/PMMA) blends are obtained from solution after evaporation of the solvent. The degree of mixing of the two polymers is studied using scanning electron microscopy after selective elimination of the PS phase. Using star‐shaped instead of linear PS, an important degree of mixing is observed. This must be attributed to difficult reptation of the star‐shaped chains due to the high number of entanglements between star‐shaped PS and PMMA compared to the entanglements between linear PS and PMMA.