CH2 radical

Usually, the decrease in conductivity during chemisorbtion of alkyl radicals on semiconductor oxides of n-type at elevated temperature has a reversible nature. However, the effect value under the same conditions depends on the chemical nature of adsorbent. For example, the following adsorbent activity row can be deduced if the oxides being studied are arranged in a chemisorbtion-induced conductivity descent order. In case of, say, CH2-radicals, the other experimental conditions being the same, we obtain ... 

Reduction of NO is promoted by N atoms and CH2 radicals formed by ethylene decomposition, but it is much less important as compared to oxidation. 

Looking at the signal at m/e = 14 (Fig. 5(b)), which corresponds to the methylene radical produced in the same reaction, we notice that its threshold is at around 9.5 eV, which is somewhat red-shifted with respect to the adiabatic IE of triplet CH2 (IE = 10.386eV).50 This is not surprising since the CH2 radical product is internally hot, but it could also... 

On the other hand, radicals are undoubtedly involved in the photodegradation of PVC under some experimental conditions. Recent ESR studies have provided evidence for the formation of alkyl and allyl-type radicals during the low-temperature UV irradiation of the polymer (43,72,87). Peroxy radicals were also observed when molecular oxygen was present (43,87). Other ESR work has shown convincingly that the radical -CHCI-CH2-CH-CH2-CHCI- results from the irradiation of PVC at liquid-nitrogen temperature (61,93) and is converted into a -CH2-CC1-CH2- radical at -110°C (93). 

Fig. 8 Irreversible photoconversion of AvGFP. (a) Modification of the absorption spectra of AvGFP under UV light (A = 254 nm, 100 s irradiation, 12.9 mW) at 293 K, pH 8.0, showing the increase in anionic B band (maximum at 483 nm). (b) Proposed Kolbe mechanism for Glu222 decarboxylation through transient formation of a CH2 radical intermediate. Reproduced with permission from [166]...

The electric energy consumption was about 0.2 eV/mol H2. The possible role of chain-radical reactions involving H, CH3, and CH2 radicals generated by the plasma is discussed... 

The first step regards the reorganizational energy of the NH2 CH2 radical—it is known (Table 12.3). 

The —O—CH2 radicals formed by C—C breakage apparently recombine or abstract since no formaldehyde is detected in the absence of air. In any case, the yield is low in comparison to Reactions 1 and 2. 

The supposedly high cross sections for these reactions again require the existence of very loose transition states, much looser than those which are normally found for H-atom abstractions and it again appears quite reasonable to suppose that ionic states are contributing to the formation of very loose transition states. In the preceding sequence (eq. T) we see that the transition states A and B are very close to the polar states (eq. B) we have already proposed for CH3 recombination. If our suggestions for the latter are correct then they would automatically imply a fairly high cross section for the insertion reactions of CH2 radicals. 

The similar behavior of CO and CHO in reactions 0 + C2H and O + C2H provided confirmation to the conclusion that the same CH2 radical is formed by the primary steps of these reactions. In an earlier work4 (using discharge in H20 vapor involving less 02 molecules in the reaction vessel), the formation of CH2 in a reaction of O atoms with C2H4 was evident from its effect on a tellurium mirror. The reaction yielding... 

There are four surface species in this reaction mechanism (1) CH(s), a surface carbon atom bonded to a hydrogen (2) C (s), a reactive surface carbon radical, namely species 1 with the hydrogen stripped away (3) CM(s), a surface CH3 group atop a surface carbon atom and (4) CM (s), a surface CH2 radical atop a surface carbon atom, namely species 3 with the hydrogen stripped away. 

In addition, CH2 radicals arc also produced. The energetic consideration rules out the direct dissociation path at 2660 A... 

Diazirine, as well as diazomethane and ketene, has been used as a convcm. m source of CH2 radicals. The photolysis of diazirine in the vacuum ultravo i. i leads to... 

We conclude this rather lengthy survey with a simple example the application of different basis sets to the singlet/triplet separation in the CH2 radical [66], which was used as a comparison of iV-particle space treatments in Sec. 3.2. 

If we consider as an example the addition of HC1 to ethylene, we find that whereas the propagation step for polymerization will be exothermic by about 30 kcal mole-1,146 abstraction of H from HC1 by the R—CH2- radical will be endothermic by 5 kcal mole-1. Activation energies for typical polymerization propagation steps are in the range of 6-10 kcal mole-1,147 and that for abstraction from HC1 will have to be greater than the 5 kcal mole-1 endothermicity. These data are at least indicative that radical addition of HC1 will not be favorable experimentally, it is indeed rare, but can be made to occur with excess HC1.148 With HBr the situation is different. Now the hydrogen abstraction is exothermic by about 10 kcal mole-1 and occurs to the exclusion of telomeriza-tion.149 Hydrogen iodide does not add successfully to olefins because now the initial addition of the iodine atom to the double bond is endothermic. 

The allylic radical has also by abstraction of the allylic hydrogen by the H2NC(0)CH2 radical in Thd [reaction (210] and its isomer has been produced from 6-chloromethyluracil by dissociative electron capture and also in low-tem-... 

In IMeUra, a predominating radical is the -CH2 radical (Flossmann et al. 1973, 1975a,b). A -CH2 radical (here, allylic) is also observed with Thy and 5MeCyt (Hiittermann 1970 Hiittermann et al. 1971 Dulcic and Herak 1973). Radical cations are likely precursors, while the precursor of the Thy C(6)-H--adduct that is commonly observed (Henriksen and Snipes 1970) could be the Thy radical anion (Symons 1990 see above). 

It has been known for some years that singlet methylene (CH2) radicals will insert in carbon-hydrogen bonds. Thus methylene plus ethane will give propane. This matter will be discussed later. Oxygen atoms are isoelectronic with methylene... 

The available experimental data indicate that the intermediates grafted onto the silica surface are matrix stabilized. Even when we deal with reactive intermediates, such as the (=Si-0-)3Si-CH2-CH2 radical, their thermal stability is controlled not by chemical processes involving the surrounding matrix, but by the rate constants of the intramolecular isomerization and decomposition reactions ... 

Friedman and Shechter (27) found that substituted formamides undergo similar reactions with olefins in the presence of peroxides at elevated temperatures to give products resulting from the addition of both CON (CH3)2 and HCON (CH3)CH2 radicals to the olefins,... 

Table 1.1 shows one substituent effect that influences the stability of radicals. The dissociation enthalpies of reactions that lead to R—CH2 radicals are listed. The substituent R varies from C2H5 through H2C=CH—(vinyl substituent, vin) to C6H5— (phenyl substituent, Ph). The dissociation enthalpy is greatest for R = H. It can also be seen that a radical center is stabilized by 12 1 kcal/mol by the neighboring C=C double bond of an alkenyl or aryl sub-... 

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