Atoms sources

An important question to consider when using a flame as an atomization source, is how to correct for the absorption of radiation by the flame. The products of combustion consist of molecular species that may exhibit broad-band absorption, as well as particulate material that may scatter radiation from the source. If this spectral interference is not corrected, then the intensity of the transmitted radiation decreases. The result is an apparent increase in the sam-... 

Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

A bis(chelate) structure was found for the closely related germylene [MeC(NPr )2]2Ge, which was also made from GeCl2(dioxane) and 2 equivalents of the lithium amidinate (colorless crystals, 81%). The same synthetic approach was used to make bis(amidinato) metal dichlorides of silicon and germanium in high yields (83-95%). Rapid oxidative addition of chalcogen atom sources (styrene sulfide and elemental Se) to the germylene derivatives resulted in a series... 

C-labeled experiments were undertaken to establish the atom source of the thiophene ring (Equations 21 and 22). The methyl group and the C-3, CA, and C-5 carbon atoms of the ring come from the (thioacyl)methylene system of 27, whereas C-2 comes from the methyl group of the 5-acetyl substituent. 

Scheme 21. Reactions of 1 with chalcogene-atom sources.

Fig. 13.16a. As an atom source, a magneto-optical trap (MOT) for cold Cs-atoms was used. The fluorescence of MOT atoms around the MNF was detected by the measurement of fluorescence photons with an avalanche photodiode connected to one end of the fiber. Signals are accumulated and recorded on a PC using a photon-counting.

Rychnovsky et al. considered the formation of achiral conformers from chiral molecules and trapping the prochiral radical with a hydrogen atom donor based on memory of chirality (Scheme 12) [41], The photo-decarboxylation of optically active tetrahydropyran 40 leads to an intermediate 43, which now does not contain a stereocenter. If the intermediate 43 can be trapped by some hydrogen atom source before ring inversion takes place, then an optically active product 41 will be formed. This is an example of conformational memory effect in a radical reaction. It was reported that the radical inversion barrier is low (< 0.5 kcal/mol) while the energy for chair flip 43 44 is higher (5 to... 

Addition of radicals to carbon-carbon or carbon-heteroatom multiple bonds followed by the trapping of resulting radicals with a hydrogen atom source... 

Model for the formation of manganese nodules under oxio oonditions by means of adsorption onto sediment grains Me = metal atom. Source-. From Lyle, M., et al. (1984). Geochimica et Cosmochimica Acta, 48, 1705-1715. 

The measurement of very low levels of environmental pollutants is becoming increasingly important. The determination of lead, a cumulative toxin, is a good example. The current maximum allowable concentration of lead in British drinking water, before it enters the distribution network, is SO ng ml [29]. Although electrothermal atomization atomic-absorption spectrometry (AAS) can be used to measure this and lower concentrations, it is slow and requires considerable effort to ensure accurate results. Flames can provide simple and effective atom sources, but, if samples are aspirated directly, do not provide sufficient sensitivity. Thus, if a flame is to be used as the atom source, a preconcentration step is required. 

Preliminary efforts to examine the mechanism of C-H amination proved inconclusive with respect to the intermediacy of carbamoyl iminoiodinane 12. Control experiments in which carbamate 11 and PhI(OAc)2 were heated in CD2CI2 at 40°C with and without MgO gave no indication of a reaction between substrate and oxidant by NMR. In Hne with these observations, synthesis of a carbamate-derived iodinane has remained elusive. The inability to prepare iminoiodinane reagents from carbamate esters precluded their evaluation in catalytic nitrene transfer chemistry. By employing the PhI(OAc)2/MgO conditions, however, 1° carbamates can now serve as effective N-atom sources. The synthetic scope of metal-catalyzed C-H amination processes is thus expanded considerably as a result of this invention. Details of the reaction mechanism for this rhodium-mediated intramolecular oxidation are presented in Section 17.8. 

Conditions were found under which simple inorganic oxorhenium species act as efficient olefin epoxidation catalysts. This has been achieved by simply replacing aqueous H2O2 with BTSP as an oxygen atom source in the epoxidation. Representative substrates include fairly unreactive olefins and/or progenitors of sensitive epoxides . For example, terminal olefins can be efficiently converted to epoxides (equation 40, Table 9). 

The reaction of CBr4 with potassium is reported to generate free C atoms and the rate constants for reaction with methane, ethylene, and benzene have been reported. The reaction of nitrogen atoms with CN radicals has also been used as a C atom source. Carbon atoms have also been produced by passing organics through a microwave discharge. ... 

A large part of the success of the combination of FI and atomic spectrometry is due to its ability to overcome interference effects. The implementation of some pretreatment chemistry in the FI format makes it possible to separate the species of the analyte from the unwanted matrix species e.g. by converting each sample into a mixture of analyte(s) and a standard background matrix, designed not to interfere in the atom formation process and/or subsequent interaction with radiation in the atom cell). Often such separation procedures result also in an increased analyte mass flux into the atom source with subsequent improvements in sensitivity and detection limits. 

C. Alternative hydrogen atom sources and chain carriers 155... 

C. Alternative Hydrogen Atom Sources and Chain Carriers... 

A number of other thiocarbonyl derivatives have been used [222] (sec below), as well as various hydrogen atom sources (tin and silicon hydrides,... 

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