Electron capture aromatic hydrocarbons

By assuming that the electron capture coefficient is equal to the equilibrium constant for the reaction of thermal electrons with aromatic hydrocarbons, the electron affinity can be obtained by measuring the response as a function of temperature. The equilibrium constant Keq is related to the electron affinity of the molecule by... 

In Fig. 8.15, the values of hvCT are plotted against the half-wave potentials of the acceptors measured in acetonitrile. A near-linear relation is observed. Peover also obtained, by an electron capture method, the values of EA of aromatic hydrocarbons in the gas phase and confirmed a near-linear relation with the reduction potentials in the solution phase. 

Lopez-Avila et al. [107] showed that microwave-assisted extraction of pesticides and polycyclic aromatic hydrocarbons from soil is a viable alternative to Soxhlet extraction and needs a smaller sample volume and extraction time [108,109]. These techniques have also been compared in the case of chlorophenols. Lopez-Avila et al. compared microwave-assisted extraction with electron capture gas chromatography to ELISA for the determination of polychlorinated biphenyls in soils. Both techniques are applicable to field screening and monitoring applications. Microwave-assisted extraction [111, 112] and solid-phase microextraction [113] have been applied to the extraction of pesticides from soil. It was observed by these and other workers [114] that the selectivity of microwave-assisted extraction is highly dependent on the soil composition. 

P. Grimsrud and C. Valkenburg, New schemes for the electron capture sensitization of aromatic hydrocarbons, J. Chromatogr., 302 243-256(1984). 

A radical anion of an aromatic hydrocarbon was implicated as early as 1866, when Berthelot obtained a black dipotassium salt from naphthalene and potassium [41]. This reaction must have proceeded via the naphthalene radical anion as a more or less fleeting intermediate. Again, Schlenk and co-workers captured the essence of such an intermediate. In the case of anthracene they noticed the existence of two different species, a purple dianion and a blue transient species with a banded spectrum [42]. They identified this intermediate as a monosodium addition product which contains trivalent carbon . Further details were revealed only with the advent of electron paramagnetic resonance spectroscopy. 

In aromatic hydrocarbons, some substituted alkenes, dienes, substituted acetylenes and ketones, one half of the n orbitals are empty and an electron can easily be placed in these antibonding orbitals. The capture of an electron by the acceptor molecule is an exothermic process because the energy of the antibonding orbitals lies below the level of the ionization potential of the acceptor radical anion. Many radical anions formed from unsaturated molecules are themselves stable they do not decompose and may exist indefinitely under suitable experimental conditions [182a],On the other hand, they react easily with other molecules. 

Wang and Charles Han calculated the electron affinities of aldehydes and ketones by using the parameterized Huckel theory. Eight parameters were used to calculate the electron affinities of 16 compounds with a deviation of only 0.05 eV. However, some of the data were not published until the 1970s [35]. By measuring relative electron capture coefficients and scaling to the acetophenone data, more precise electron affinities could be obtained. This was further support for the validity of the ECD model. M. J. S. Dewar reproduced the experimental electron affinities of aromatic hydrocarbons using the MINDO/3 method and calculated Ea from reduction potentials [36]. 

Fewer than 300 Ea for organic molecules have been determined in the gas phase. The majority of the Ea have been determined by the ECD and/or TCT methods. The direct capture magnetron, AMB, photon, and collisional ionization methods have produced fewer than 40 values. Only the Ea of p-benzoquinone, nitrobenzene, nitromethane, azulene, tetracene, and perylene have been determined by three or more methods. Excited-state Ea have been obtained by each of these methods. Half-wave reduction potentials have determined the electron affinities of 50 aromatic hydrocarbons. The electron affinities of another 50 organic compounds have been determined from half-wave reduction potentials and the energies of charge transfer complexes. It is a manageable task to evaluate these 300 to 400 Ea. 

Because of the large scale dilution of contaminants in the aquatic matrices, concentrations of many organic pollutants are below the detection limits of standard analytical and sampling methods. Thus, gas chromatography with specific detection methods such as electron capture detector and HPLC has been frequently used for analysis of pesticides and polycyclic aromatic hydrocarbons in water and biological samples. 

Once you have the extracted and preconcentrated organic analytes, you may have to perform additional cleanup, for example, by running them through a column of adsorbent packing material such as silica or alumina. Then chromatography is most often used for measurement. Pesticides are commonly determined by gas chromatography with electron capture detection or GC-MS. A nonpolar capillary GC column is used. Trace PCB and polycyclic aromatic hydrocarbon (PAH) determinations can be done using HPLC with UV detection. 

An experimental study has been carried out with peat samples from the forest area of Brunei Darussalam. We should note here that the measurement of emission products requires comprehensive analytical equipment. Hydrocarbons (C1-C4) are determined by gas chromatography with flame ionization detection (GC/FID), CO2 and O2 are analyzed by gas chromatography with thermal conductivity detection (GC/TCD), and CO, by gas chromatography with electron capture detection (GC/ECD). Aldehydes and polynuclear aromatic hydrocarbons (PAHs) are determined by gas chromatography with mass spectrometry (GC/MS). 

Priego-Capote, F., Luque-Garcia, J. L., and Luque de Castro, M. D., Automated fast extraction of nitrated polycyclic aromatic hydrocarbons from soil by FMAS extraction prior to gas chromatography-electron-capture detection, J. Chromatogr. A, 994, 159-167, 2003. 

Here, A is the hydrocarbon to be reduced, e is a free electron, and A is the radical anion of A. The exothermicity of reaction 1 is referred to as the electron affinity of A, and its absolute value is equal to the ionization potential of A" . Positive values of electron affinity imply that aromatic hydrocarbons are able to capture free electrons, a phenomenon to be described later. 

The kinetics of disproportionation is conveniently studied by flash photolysis, A flash of visible light leads to the photoejection of electrons from radical anions or dianions (II). Consider an equilibrated system involving an aromatic hydrocarbon, its radical anion, and its dianion. A flash of light ejects electrons from the dianions and radical anions to convert the dianions into radical anions and the radical anions into the parent hydrocarbon. The ejected electrons are rapidly captured, mainly by the hydrocarbons this process converts the hydrocarbons into radical anions in less than a few milliseconds. The following cases should be considered ... 

Studies of gaseous electron capture taking place in a plasma were reported by Wentworth et al. (18). The equilibrium established in a plasma between e, A, and A " allowed the determination of the absolute electron affinities of the aromatic hydrocarbons in the gas phase. 

In reactions with organic molecules e q reacts as nucleophilic reagent it attacks molecules with low-lying molecular orbital, like aromatic hydrocarbons, conjugated olefinic molecules, carboxyl compounds, and halogenated hydrocarbons (Swallow 1982 Buxton 1982, 1987 Buxton et al. 1988). In the latter case, addition is usually followed by halide ion elimination, so the reaction can be considered as a dissociative electron capture. For instance, the reaction with chlorobenzene yields phenyl radical and chloride ion... 

M.V. Buchanan, B. Olerich, Differentiation of polycyclic aromatic hydrocarbons using electron-capture negative chemical ionization, Org. Mass. Spectrom. 1984,19, 486. 

Buchanan, M.V. Olerich, G. Differentiation of Polycyclic Aromatic Hydrocarbons Using Electron Capture N a-tive Chemical lorrization. Org. Mass Spectrom. 1984,79,486-489. 

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