Thermal abstraction reactions atoms

Grafting reactions onto a polymer backbone with a polymeric initiator have recently been reported by Hazer [56-60]. Active polystyrene [56], active polymethyl methacrylate [57], or macroazoinitiator [58,59] was mixed with a biopolyester polyhydroxynonanaate [60] (PHN) or polybutadiene to be carried out by thermal grafting reactions. The grafting reactions of PHN with polymer radicals may proceed by H-abstraction from the tertier carbon atom in the same manner as free radical modification reactions of polypropylene or polyhy-droxybutyratevalerate [61,62]. 

Another important characteristic of radical abstraction reactions is the force constants of the ruptured and the generated bonds. The dependence of the activation energy for the reactions of the type R + R X > RX + R1, where X = H, Cl, Br, or I, on the coefficients Ai and Af was demonstrated experimentally [17]. It was found that parameter re = const in these reactions, while the square root of the activation energy for a thermally neutral reaction is directly proportional to the force constant of the ruptured bond. The smaller the force constant of the C—X bond, the lower the Ee0, and the relationship Feo12 to A(1 I a) 1 is linear (see Figure 6.4). The same result was also obtained for the reactions of hydrogen atoms with RC1, RBr, and RI [17]. 

The activation energy of radical abstraction is influenced by the so-called triplet repulsion in the transition state. This influence is manifested by the fact that the stronger the X—R bond towards which the hydrogen atom moves in the thermally neutral reaction X + RH, the higher the activation energy of this reaction. The triplet repulsion is due to the fact that three electrons cannot be accommodated in the bonding orbital of X—C therefore, one electron... 

They were used for the calculation of the activation energies for isomerization of several peroxyl radicals. Peroxyl radical isomerization involving the formation of a six-membered activated complex is energetically more favorable the activation energy of a thermally neutral reaction Ee is 53.2 kJ mol-1. For the seven-membered transition state, the Ee0 value (54.8 kJ mol-1) is slightly higher. The calculated hrc parameter for the six-membered transition state (13.23 (kJ mol-1)172) is close to the bre value (13.62 (kJ mol-1)172) for the transition state of the bimolecular H atom abstraction from the aliphatic C—H bond by the peroxyl radical. Therefore, the kinetic parameters for isomerization are close to those for bimolecular H-atom abstraction by the peroxyl radical. This allows the estimation of the kinetic parameters for peroxyl radical isomerization. Relevant results of calculation via Eqns. (6.7, 6.8,... 

The alkyl radical may also dissociate thermally to form an alkene and a smaller alkyl radical. The mechanism that is initiated by these reactions is chain propagating rather than chain branching and for this reason the overall oxidation rate of the fuel decreases. Also there is a change from OH to HO2 as the main chain carrier, and as we have seen, the HO2 radical is much less reactive than OH. The HO2 radical is formed both from alkyl + O2 hydrogen abstraction reactions such as (R69) and from recombination of hydrogen atoms with O2, H + O2 + M HO2 + M (R5). Under lean conditions any hydrogen atoms formed will primarily react with oxygen. At intermediate temperatures the reaction H + O2 O + OH (Rl) is still too slow to compete with (R5). 

The accurate determination of rate constants for the reactions of 19F atoms is often hampered by the presence of reactive F2 and by the occurrence of side reactions. The measurement of the absolute concentration of F atoms is sometimes a further problem. The use of thermal-ized 18F atoms is not subject to these handicaps, and reliable and accurate results for abstraction and addition reactions are obtained. The studies of the reactions of 18F atoms with organometallic compounds are unique, inasmuch as such experiments have not been performed with 19F atoms. In the case of addition reactions, the fate of the excited intermediate radical can be studied by pressure-dependent measurements. The non-RRKM behavior of tetraallyltin and -germanium compounds is very interesting inasmuch as not many other examples are known. The next phase in the 18F experiment should be the determination of Arrhenius parameters for selected reactions, i.e., those occurring in the earth s atmosphere, since it is expected that the results will be more precise than those obtained with 19F atoms. 

While electron-transfer processes are common in inorganic photochemistry, excited-state atom transfer is limited to a small class of inorganic complexes. For U022 , the diradical excited state ( U-OO is active in alcohol oxidation (2). The primary photoprocess is hydrogen atom abstraction by the oxygen-centered radical. Photoaddition to a metal center via atom transfer has been observed for binuclear metal complexes such as Re2(CO)io (3-5). The primary photoprocess is metal-metal bond homolysis. The photogenerated metal radical undergoes thermal atom-abstraction reactions. Until recently, atom transfer to a metal-localized excited state had not been observed. 

The pyrolysis Is Initiated by thermal fission of the CHi-0 bond (reaction 1). The resulting oxynaphthyl radical and methyl radical abstract H-atoms from the parent forming naphthol, CH% and the methylene-naphthylether radical (reactions 2 and 3). 

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