Bond homolysis reactions

Re—Re, Re—M, and Re—C Bond Homolysis Reactions Photochemistry of Re(I) Diimine Tetracarbonyl Complexes Photochemical Ligand Substitution Reaction of/ac-[Re(Diimine)... 

For Red) dimers with a Re—Re bond, photoexcitation induces homolysis of the Re—Re bond. The two rhenium(O) metal centers in Re2(CO)io each donate one electron from their d 2 orbitals to make a a M—M bond. Upon photoirradiation, the bonding a-orbital (ob) electron is excited into the antibonding a-orbital (qz ), which leads to metal-metal bond cleavage. It has been also reported that a CO ligand elimination proceeds competitively with the metal-metal bond homolysis reaction. When a cyclohexane solution of Re2(CO)io was irradiated by the 355-nm laser light, the ratio of the reaction quantum 5nelds of Re—Re bond... 

Pulse radiolysis has been used to study the transient formation and decomposition of cobalt-alkyl bonds in aqueous solution in the same manner as it has been used for chromium alkyls. And as for chromium alkyls, bond homolysis is a major decomposition pathway (28). For bond formation reactions, pulse radiolysis shows that they are assisted by increases in pressure. This feature results from the homolysis having a larger activation volume than the bond formation reaction, resulting in a significantly negative overall reaction volume for the process (29). In general for all of these metal-alkyl bond homolysis reactions of the aquo complexes, steric hindrance facilitates the reaction. Ligand effects also play a role, but the factors involved are more subtle. 

Kinetic data for the decomposition of mono- and diperesters of oxalic acid are given in Table 109. The three benzyl mono-peresters did not give a linear plot with either or or ct substituent constants Instead, the response of the substituents was intermediate between these two constants. If a three-bond homolysis reaction... 

Activation parameters for monoperoxycarbonates (Table 111) indicate a one-bond homolysis reaction. In comparison to di-t-butylperoxyoxalate where multibond homolysis occurs, the rate coefficient for di-f-butyl monoperoxycarbonate is 10 times slower at 45 °C . The rate of decomposition of di-t-butyl diperoxy-... 

Kinetic data for thermal decomposition of the related dialkyl peroxydicarbonates are given in Table 112. Variation of the substituent groups R has little effect on the rate coefficients or the activation energy. In addition the activation energies are in the range of those reported for benzoyl peroxides. This suggests a one-bond homolysis reaction. Activation energies for dialkyl peroxydicarbonates with... 

Therefore, first-order, decomposition rates for alkyl hydroperoxides, ie, from oxygen—oxygen bond homolysis, are vaUd only if induced decomposition reactions... 

Decomposition of Thiols. Thiols decompose by two principal paths (i43— i45). These are the carbon—sulfur bond homolysis and the unimolecular decomposition to alkene and hydrogen sulfide. For methanethiol, the only available route is homolysis, as in reaction 29. For ethanethiol, the favored route is formation of ethylene and hydrogen sulfide via the unimolecular process, as in reaction 30. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

The chemistry of [Rh(OEP)h in benzene is dominated by Rh—Rh bond homolysis to give the reactive Rh(Il) radical Rh(OEP)-. This contrasts with the reactivity of fRh(OEP)] in pyridine, which promotes disproportionation via the formation of the thermodynamically favorable Rh(IlI). ct complex [RhjOEPKpy) ] together with the Rh(l) anion, Rh(OEP)J The hydride complex Rh(OEP)H shows NMR chemical shift changes in pyridine consistent with coordination of pyridine, forming Rh(OEP)H(py). Overall, solutions of Rh(OEP)l in pyridine behave as an equimolar mixture of [Rh(OEP)(py ) and (Rh(OEP). For example, reaction... 

Hydroxyl radical is a strong indiscriminate outer-sphere oxidant (generating OH ) and H-atom abstractor (generating H2O) [Huie and Neta, 1999]. Simple Fe porphyrins are known to promote 0-0 bond homolysis in reaction with H2O2 [Watanabe, 2000]. Because of its high reactivity, once generated, "OH probably reacts with the... 

It is seen that the values of kd are very close. Hence, the reaction of POOH with the C—H bond is not the main initiation reaction. If the breakdown is a monomolecular process, the rate of O—O bond homolysis in polymer must be close to that in the gas phase. 2,2-Dimethylethyl hydroperoxide breaks down in the gas phase with a rate constant of 1.6 x 1013 exp(— 158/i 7) = 5.3 x 10 x s 1 (398 K, [4]), that is, by four orders of magnitude more slowly than in polymer. Hence, the decomposition reactions in the polymers are much faster than the monomolecular homolysis of peroxide. Decomposition reactions may be of three types (see Chapter 4), such as the reaction of POOH with a double bond... 

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