Cage recombination

Photodissociation of a linear triatomic such as [85, 86] or Hgl2 [8] to produce a vibrationally excited diatomic, or cage recombination of a photodissociated diatomic such as I2 [78, 81] are classic model simple systems for reaction dynamics. Here we discuss tire Hgl2—>HgI + I reaction studied by Hochstrasser and co-workers [87, 88 and 89]. 

Approximately 5% of radicals undergo cage recombination when dicumyl hyponitrite (35) is decomposed in bulk MMA or S at 60 CC.72 Dicumyl peroxide, the product of cage recombination is likely to be stable under the conditions where hyponitrites are usually employed. Nonetheless, its formation is a concern since... 

A slow rate of p-scission also means that the main cage recombination process will be cage return to reform the peroxydicarbonate. Dialkyl peroxides are typically not found amongst the products of peroxydicarbonate decomposition. In these circumstances, cage recombination is unlikely to be a factor in reducing initiator efficiency. 

The low conversion initiator efficiency of di-r-butyl pcroxyoxalatc (0.93-0.97)1-1 is substantially higher than for other peroxyeslers [/-butyl peroxypivalale, 0.63 /-butyl peroxyacetate, 0.53 (60 °C, isopropylbenzene)195]. The dependence of cage recombination on the nature of the reaction medium has been the subject of a number of studies. 12I,1<>0 20CI The yield of DTBP (the main cage product) depends not only on viscosity but also on the precise nature of the solvent. The effect of solvent is to reduce the yield in the order aliphatic>aromatie>protic. It has been proposed199 that this is a consequence of the solvent dependence of p-scission of the f-butoxy radical which increases in the same series (Section 3.4.2.1.1). 

Three types of model study have been performed. The first approach has been to decompose a mixture of two initiators (/.< . one to generate radical A, the other to generate radical B). With this method experimental difficulties arise because the two types of radical may not be generated at the same rate and because homotermination products from cage recombination complicate analysis. 

A second approach has been to use an unsymmetrical initiator which allows the two radicals of interest to be generated simultaneously in equimolar amounts.175 In this case, analysis of the cage recombination products provides information on cross termination uncomplicated by homotermination. Analysis of products of the encounter reaction can also give information on the relative importance of cross and homotermination. However, copolymerization of unsaturated products can cause severe analytical problems. 

The mode of fission of some azo compounds into alkyl radicals and nitrogen has been studied by Pryor and Smith<8) using the following postulates (1) A molecule that decomposes by a concerted scission of both C—N bonds will not undergo cage return and will have a rate constant independent of viscosity (2) a molecule that decomposes by a stepwise scission of the C—N bonds can undergo cage recombination and the rate constant for decomposition will decrease with solvent viscosity increase provided that the lifetime of the radicals produced by the initial homolysis is of the same order... 

The chain termination is a result of tertiary alkylperoxyl radical recombination in the solvent cage. The values of the rate constants for chain termination through the disproportionation of tertiary peroxyl radicals are collected in Table 2.15. They vary in the range 103 to 105 L mol 1 s 1 at room temperature. The probability of a pair of alkoxyl radicals to escape cage recombination is sufficiently higher than that of cage recombination. The values of rate constants of the reaction 2 R02 > 2 RO + 02 measured by the EPR technique are presented in Table 2.16. 

The thermal decomposition of azoalkanes bearing geminal a-cyano and a-trimethylsiloxy groups has been the subject of a report. The symmetrical compound (107) decomposes near room temperature to afford entirely C—C dimers, whereas the unsymmetrical azoalkane (108) requires heating to 75 °C. A NMR product study of photolysed (107) in the presence of TEMPO showed that the fate of caged t-butyl-l-trimethylsiloxy-l-cyanoethyl radical pairs is disproportionation (17%), cage recombination (20%), and cage escape (63%). 

The final destiny of IRP depends on the nature of the solvent. In DMSO, the main process is an inner-cage recombination (route a), a minor process consists of IRP disintegration after its diffusion into volume (route b) ... 

Few examples have been reported (5, 8, 9, 10, 12, 24) of cage recombination of simple alkoxy or acyloxy radicals to form O—O bonds in isolable molecules. This paper explores further the implications of the observed (17, 22, 23) scrambling of label seen in acetyl peroxide carbonyl-18O recovered after partial decomposition. 

Mechanism A clearly predicts the formation of more double-labeled oxygen, in this case a 36/34 ratio of 0.0149, calculated from statistical considerations. Mechanism B, involving cage recombination of acetoxy radicals, predicts a more random distribution and a 36/34 ratio of 0.00828, closer to the experimentally determined ratio of 0.00835. 

They further noted (2) that the over-all rate constant for disappearance of acetyl peroxide decreases monotonically with increasing solvent viscosity. The attractive hypothesis (19, 22, 23) that the observed rate decrease with increased viscosity in this homologous series of hydrocarbon solvents reflects the increased importance of cage recombination of acetoxy radicals in the more viscous solvents is subject to further test from the data of this paper. 

Table III and the plot of Figure 3 compare values of kd (obs.) and kd (calc.) in several solvents. The values of kd (obs.) were taken from the data of Szwarc (7) and Eirich (2) as well as from our laboratory and involve an extrapolation from 60° to 80°C. for the data of Eirich. This was done by making the crude approximation that AG varies with solvent in the same way at the two temperatures. A close parallel exists between the observed and calculated rates. This suggests that the assumption equating ki and kg is not bad and provides a firmly established case in which the dependence of the rate of decomposition of a radical initiator on solvent viscosity in a homologous series of solvents results from the varying importance of cage recombination in the various solvents. Pryor and Smith (19) using data from p-nitrophenylazotriphenylmethane decompositions have suggested that a general criterion for radical cage...

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