Phenylperoxy radical

In aromatic combustion flames, cyclopentadienyl radicals (c-CgHj ) can be precursors for PAH formation. " At high temperatures, benzene is oxidized by reaction with an oxygen molecule to yield phenylperoxy (C6H5O2 ) radical, via the initial formation of the phenyl radical (by C-H bond cleavage) and then the rapid addition of O2 (reaction 6.16). After expulsion of CO from phenylperoxy radical, a resonance-stabilized cyclopentadienyl radical (c-CgHg ) is formed (reaction 6.16). 

Although greatly simplified, this model demonstrates the importance of phenylperoxy radical (C6H5OO ) as a reactive intermediate and accounts for some of the major combustion products. However, several other mechanistic intermediates (C3, C4, and C5 hydrocarbons) were also observed. 

Consequently, the benzene oxidation mechanism was further developed by considering additional decomposition and oxidation steps. Sethuraman et al. proposed that phenyl radical decomposition can occur by either of two key pathways (3-scission of phenyl radical or by breakdown of the phenylperoxy radical formed by the oxidation of phenyl radical (Fig. 9). Using PM3 calculations,which were ultimately verified by DFT studies,Carpenter predicted that another species, 2-oxepinoxy radical (3 in Fig. 9b), is an important intermediate due to its relative stability, formed via a spirodioxiranyl intermediate (2 in Fig. 9b) from phenylperoxy radical. Pathway A in Fig. 9b is the thermodynamically preferred pathway at temperatures increasing up to 432 K, while pathway B has an entropic benefit at higher temperatures. While pathway B essentially matched the traditional view of benzene combustion, pathway A introduced a new route for phenylperoxy radical, which could resolve discrepancies observed using previous models. 

This supposition was validated by experimental studies that demonstrated the prevalence of different ROS at different temperatures. Using CRDS, Yu and Lin studied the reaction of phenyl radical and oxygen, noting that phenylperoxy radical was the only adduct formed at temperatures ranging up to 473 Venkat et al. completed flow reactor studies of benzene combustion at 1200K and identified phenoxy radical as a key intermediate. 

A subsequent study examined phenylperoxy radical in greater detail. Fadden et identified five possible unimolecular decomposition pathways for phenylperoxy radical (Fig. 10) via oxygen atom loss to form phenoxy radical (Fig. 10, route A), via a dioxiranyl radical species (Fig. 10, route B), via a dioxetanyl radical... 

Fig. 10 Potential unimolecular reaction pathways for phenylperoxy radical. Adapted from Reference 129b.

Phenylperoxy radical has similarly been a topic of experimental and theoretical interest. Tokmakov et al. " calculated a potential energy surface for phenyl radical and O2 using ab initio G2(MP2) calculations. Weisman and Head-Gordon used time-dependent density functional theory (TD-DFT) calculations to examine the effect of substituents on the phenylperoxy radical s UV-vis absorption spectrum. " Lin and Mebel used ab initio methods to study the phenoxy radical -f O-atom reaction. "... 

CRDS. Tonokura et al. used CRDS to study the visible absorption spectrum of the phenyl radical, as well as the kinetics of its reaction with O2. Krauss and Osman examined the UV absorption spectra of vinylperoxy radical (H2C=CH02 ) and phenylperoxy radicals. 

Phenylperoxy radical, originally assumed to be a factor in low-temperature combustion only, has actually been shown to play a substantial role in dictating the overall combustion trends of benzene. Just as the isomerizations and eliminations of the alkylperoxy radicals significantly affected their overall combustion pathways, rearrangements and other intramolecular pathways available to phenylperoxy radical similarly impact the overall progress of benzene combustion. This knowledge can be extrapolated to more complex aromatic species. 

While most studies have focused on the pyrolytic unimolecular decomposition of these monoheteroaromatic compounds, our group has explored their oxidative decomposition. As with benzene, where phenylperoxy radical plays a major role in dictating oxidation pathways, we hypothesize that the peroxy radicals derived from heteroaromatic rings are reactive species of considerable interest for combustion and atmospheric reactions. 

High-molecular-weight aldehydes (RCHO, R > H) ALD2 Phenylperoxy radical PH 02... 

There are indications that cr-radicals, such as the phenyl radical may not react with oxygen as readily as the alkyl radicals (Hay, 1967). Attempts to form the phenylperoxy radical in the rotating cryostat have failed, but the results were insufficiently conclusive to determine whether, or not, the failure was due to a low reactivity of the phenyl radical towards oxygen addition,... 

The overall pathways of benzene oxidation and the decompositions of possible intermediates have been well characterized via theoretical methods. Thus far, we have discussed these species mainly in the context of their oxidation mechanisms, but phenylperoxy and phenoxy radicals have also been investigated as individual experimental targets. 

As might be expected not all attempts to prepare and trap a given radical have been successful. Of particular interest amongst the failures are the ammonium radical, NH4, the phenylperoxy and the alkoxy radicals. 

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