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Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow

While the first Electron Paramagnetic Resonance (EPR) studies regarding the effects of oxidation on the structure and stability of carbon radicals date ...

The Hebrew University of Jerusalem

Palladium 15-20 nm particles stabilized by a Keggin-type polyoxometalate were prepared by reduction of K(5)PPdW(11)O(39) with H(2). The nanoparticles were shown to be effective catalysts for Suzuki-, Heck-, and Stille-type carbon-carbon coupling and carbon-nitrogen coupling reactions of bromoarenes in aqueous media. Chloroarenes were also reactive in reaction media without solvent. [reaction: see text]

Carbon-carbon and carbon-nitrogen coupling reactions catalyzed by palladium nanoparticles derived from a palladium substituted Keggin-type polyoxometalate.

The Kishon River, the second largest coastal river in Israel, has been severely polluted for several decades. Sediments from upstream and downstream sites of the river were analyzed, lipid-extracted and evaluated for phenanthrene uptake. Total polycyclic aromatic hydrocarbon (PAH) concentration in downstream sediments was 299 microg kg(-1), consisting mostly of petrogenic-derived PAHs. Downstream sedimentary lipids were found to be dominated by fresh and decomposed petroleum-derived n-alkanes. The total PAH concentration in upstream sediments was 173 microg kg(-1), consisting mostly of pyrogenic-derived PAHs, whereas lipids from these sediments were mostly vegetation-derived. Spectroscopic data suggested an exceptionally high aromatic content in downstream humic acid, which originated from PAHs attached to its structure. Sorption data suggested that upstream sedimentary cuticle-derived lipids function as a sorption domain, while downstream sedimentary lipids, consisting of shorter-chain-length petroleum-derived alkanes, compete with phenanthrene for sorption sites.

Persistent organic pollutants and sedimentary organic matter properties: a case study in the Kishon River, Israel.

Methylation of polysulfides [(NH4)2Sn)] by reaction with CF3SO3CH3 followed by separation of the produced dimethylpolysulfides by liquid chromatography and subsequent highly accurate stable isotope analysis by a continuous-flow isotope ratio mass spectrometer shows that polysulfide anions in an aqueous solution exchange isotopes with the other sulfur species in the system. It demonstrates for the first time that polysulfide anions are 34S-enriched in equilibrium relative to total sulfur as a function of their sulfur chain length.

Sulfur stable isotope distribution of polysulfide anions in an (NH4)2Sn aqueous solution.

The soil lipid fraction can play an important role in the sorption of organic compounds. In this study, the impact of the lipid fraction of freshwater- and wastewater-irrigated soils on the sorption of non- and relatively polar compounds was assessed. Lipid analyses revealed a clear difference between the two lipid fractions. The lipid extract from the wastewater-irrigated soil was consistent with mainly straight paraffinic chain materials; the lipid extract from freshwater-irrigated soil, on the other hand, exhibited stronger signals of aromatics, double bonds, ester, ether, and methyl, in addition to a smaller contribution from methylene protons. Our data suggest that lipid removal induced a stronger increase in the soil's sorption affinity for solutes capable of polar interactions such as atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and chlorotoluron (N'-(3-chloro-4-methylphenyl)-N,N-dimethylurea) as compared to phenanthrene. Moreover, the level of increase in sorption affinities due to lipid removal was much higher for the freshwater-irrigated soil than for its wastewater-irrigated counterpart, even though the level of lipids in the freshwater-irrigated soil was half that in the wastewater-irrigated one (6 vs. 11% of the total organic C). The higher level of polar functionalities, such as ether and ester moieties, in the lipid fraction from the freshwater-irrigated soil suggests that these extractable compounds compete successfully with the polar solutes (atrazine and chlorotoluron) for specific binding sites in the soil organic matter (SOM). It appears that the composition of the lipid fraction may be a key consideration in unraveling the sorption of organic molecules in soils.

The role of lipids on sorption characteristics of freshwater- and wastewater-irrigated soils.

(34)S enrichment is shown to occur during sulfurization reactions and for the first time conclusively attributed to an isotope equilibrium effect rather than selective addition of (34)S enriched nucleophiles.

Sulfur isotope fractionation during incorporation of sulfur nucleophiles into organic compounds.

Once coal is excavated it comes into contact with atmospheric oxygen and begins to undergo low temperature oxidation. The mechanism by which the molecular oxygen interacts with the coal macromolecule is suggested to occur in several steps. These steps primarily involve O(2) diffusion to the surface where physical adsorption followed by chemical adsorption takes place. The chemical adsorption forms several types of oxides that can subsequently react to form several products, primarily CO(2). It has also been suggested that some of these oxidation mechanisms might involve radical reactions. As the previous studies were conducted under conditions where significant structural changes occur it is possible that in the low temperature range (T < 100 °C) the oxidation mechanism is different. Several different rank (lignite-subbituminous-bituminous) coals were isothermally heated at 95 °C in an air atmosphere for a period of up to 6 months and samples were collected at two week intervals. The radical concentration of each sample was measured by Continuous Wave Electron Paramagnetic Resonance (CW-EPR). It is apparent that there are distinct differences between the lower rank (lignite) and the higher rank (subbituminous, bituminous) coals. The lower rank coals exhibited only carbon centered radicals with an adjacent oxygen atom and the higher rank coals exhibited only carbon centered radicals. Interestingly, the lower rank coals exhibited no change in radical concentration due to the long term oxidation treatment while the higher rank coals showed a distinct increase in the radical concentration. These findings shed new light on the complex heterogeneous low temperature oxidation reactions occurring at the coal surface.

Stable radicals formation in coals undergoing weathering: effect of coal rank.

We have examined the organic osmotic solutes content within the stratified microbial communities in an evaporitic gypsum crust found in an evaporation pond (~194 g/l total dissolved salts) of the salterns of the Israel Salt Company, Eilat. We extracted intracellular solutes from the upper three pigmented layers of the crust: a yellow-orange layer dominated by unicellular cyanobacteria, a green layer with filamentous cyanobacteria, and a layer colored red-purple by purple sulfur bacteria; dense communities of heterotrophic bacteria were present in all layers. The solutes were analyzed by Raman spectroscopy, (1)H and (13)C nuclear magnetic resonance, and HPLC. All layers contained glycine betaine as the only detectable osmotic solute; ectoine and other solutes known to be produced by many halophilic and halotolerant prokaryotes were not found. In this first attempt to assess the osmotic solute content within complex natural communities of halophilic microorganisms, the predominant role of glycine betaine as an osmolyte was established. Most heterotrophic bacteria cannot produce glycine betaine but preferentially use it when it is supplied. Presence of glycine betaine produced by the photoautotrophic members of the community, therefore, may relieve the heterotrophs from the need to synthesize other compounds at a high-energy cost.

Glycine betaine is the main organic osmotic solute in a stratified microbial community in a hypersaline evaporitic gypsum crust.

We report the discovery that a flow of CO2, N2 or He can sufficiently reduce the spin-spin interactions of specific stable carbon centered radicals by displacing the molecular oxygen in the atmosphere enabling their detection via electron paramagnetic resonance (EPR). This finding unlike other reported effects on carbon radicals occurs under STP conditions and is reversible.

Reducing the spin-spin interaction of stable carbon radicals.

Recently, the nature of the carbon radicals stabilized in various coals was characterized using Electron Paramagnetic Resonance (EPR) spectroscopy. It was demonstrated that introducing diamagnetic gases, such as He, CO2, or N2, under STP conditions to the coal surface induces the appearance of a new type of carbon surface radical. This interesting phenomenon was not observed for all coal types, which suggests that the use of EPR measurements can provide information on functional groups that exist on the carbon surface. In the current study coupling Nuclear Magnetic Resonance (NMR) with gas flow in situ EPR measurements significantly enhances the ability to characterize the nature of these radicals and the surface functional groups of coal samples. It was observed that the oxidative reaction with aliphatic groups leads to the increase in stable carbon centered radicals. In addition, there are some species of carbon centered radicals that show reversible binding to O2. This phenomena, however, is dependent on the coal rank, sample porosity and the degree of the coal sample to undergo structural changes under the LTO process. These findings shed new light onto the complex heterogeneous low temperature oxidation reactions occurring at the coal surface.

Elucidating the role of stable carbon radicals in the low temperature oxidation of coals by coupled EPR-NMR spectroscopy - a method to characterize surfaces of porous carbon materials.

Zeev Aizenshtat

Chemistry Institute

The Hebrew University of Jerusalem

Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow