Free atoms, reaction

Edwin Hart J, Henglein Amim (1985) Free radical and free atom reactions in the sonolysis of aqueous iodide and formate solutions. J Phys Chem 89 4342-4347... 

Chemical Interference. Incomplete atomization of the analyte causes chemical interference, due to the fact that atomic absorption can only occur with free atoms. Reactions in the flame which lead to the formation of thermally stable species decrease the signal. This is responsible for the depression of calcium signals in serum analysis by proteins, as well as for the low sensitivities of metals which form thermally stable oxides or carbides (Al, B, V, etc.) in flame AAS. A further example of chemical interference is the suppression of the extinction of alkaline earth metals as a result of the presence of oxyanions (OX) such as aluminates or phosphates. This well-known calcium phosphate interference is caused by the reaction ... 

High Peroxide Process. An alternative to maximizing selectivity to KA in the cyclohexane oxidation step is a process which seeks to maximize cyclohexyUiydroperoxide, also called P or CHHP. This peroxide is one of the first intermediates produced in the oxidation of cyclohexane. It is produced when a cyclohexyl radical reacts with an oxygen molecule (78) to form the cyclohexyUiydroperoxy radical. This radical can extract a hydrogen atom from a cyclohexane molecule, to produce CHHP and another cyclohexyl radical, which extends the free-radical reaction chain. 

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

Why do free-radical reactions involving neutral reactants and intermediates respond to substituent changes that modify electron distribution One explanation has been based on the idea that there would be some polar character in the transition state because of the electronegativity differences of the reacting atoms ... 

Bonds may also be broken symmetrically such that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired like all the other electrons of the atom, i.e. it does not have a partner of opposite spin. Atoms possessing odd unpaired electrons are termed free radicals and are indicated by a dot alongside the atomic or molecular structure. The chlorination of methane (see later) to produce methyl chloride (CH3CI) is a typical free-radical reaction ... 

Initiation step (Section 4.17) A process which causes a reaction, usually a free-radical reaction, to begin but which by itself is not the principal source of products. The initiation step in the halogenation of an alkane is the dissociation of a halogen molecule to two halogen atoms. 

The broadest classification of reactions is into the categories of heterolytic and homolytic reactions. In homolytic (free radical) reactions, bond cleavage occurs with one electron remaining with each atom, as in... 

A second theoretical index, and one for which there appears to be more justification in its application to free-radical reactions, is the atom localization energy. This index is a measure of the energy required to localize one electron of the 7r-electron system in the aromatic molecule at the point of attack of the radical. The formation of the intermediate adduct in a free-radical aromatic substitution may be regarded as the sum of two processes one, the localization of an electron at the point of attack and the other, the pairing of this... 

Ionic Reactions in TD/D2 Methane Mixtures. Previous investigation of the radiolysis of D2 containing small quantities of CH4 demonstrated that at low conversions all products anticipated from the H atom abstraction sequence except CH3D are absent from 125° to —196°C. and that the temperature coefficient of the rate of CH3D formation between 25° and 125 °C. is much too small for a purely atomic and free-radical reaction sequence (8). These observations are confirmed by new data presented in Table I. The new data also demonstrate the initial value of G(CH3D) is independent of temperature at 25°C. and below. 

Ionic Reactions in TD/D2 Ethane Mixtures. The data in Table III show that deuteron transfer occurs in irradiated mixtures of D2 and ethane as well. Data are shown only for temperatures (<25°C.) at which ionic reactions clearly predominate. Analysis of data concerning thermal atomic and free-radical reactions at higher temperatures will be published elsewhere in the near future. The reaction of D3 + with ethane has been observed directly (1) and postulated (2) by other workers. Both groups have proposed that the sequence initiated by deuteron transfer to ethane proceeds as follows ... 

E. W.R. Steaclie, Atomic and Free Radical Reactions, Acad. Press,... 

Lifetimes of free atoms and radicals account for the degree of interaction of these particles with an ambient medium and with each other. Due to high reaction capability of active particles in gaseous and, especially, in liquid media, their lifetimes are rather small. In gaseous phase, at small pressures these lifetimes are determined by heterogeneous recombination of these particles on vessel walls and by interaction of these particles with an adsorbed layer. At high gas pressures, the lifetimes are determined by bulk recombination and chemical interaction with ambient molecules. 

Thus, we considered a number of examples of application of the sensor technique in experiments on heterogeneous recombination of active particles, pyrolysis and photolysis of chemical compounds in gas phase and on the surface of solids, such as oxides of metals and glasses. The above examples prove that, in a number of cases, compact detectors of free atoms and radicals allow one to reveal essential elements of the mechanisms of the processes under consideration. Moreover, this technique provides new experimental data, which cannot be obtained by other methods. Sensors can be used for investigations in both gas phase and adsorbed layers. This technique can also be used for studying several types of active particles. It allows one to determine specific features of distribution of the active particles along the reaction vessel. The above experiments demonstrate inhomogeneity of the reaction mixture for the specified processes and, consequently, inhomogeneity of the... 

Many physical-chemical processes on surfaces of solids involve free atoms and radicals as intermediate particles. The latter diffuse along the adsorbent-catalyst surface and govern not only kinetics of catalytic, photocatalytic, or some heterogeneous radiative processes, but also creation of certain substances as a result of the reaction. 

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