2-oxepinoxy radical

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. 

The stability of 2-oxepinoxy radical qualified it as a target for further theoretical and experimental study. The calculations of Barckholtz et al. allowed the refinement of a feasible energetic pathway toward 2-oxepinoxy radical these DFT calculations supplemented the semiempirical work of Carpenter and also proposed a triradical intermediate between the dioxiranyl and oxepinoxy species (Fig. 9a). 

Fig. 11 Unimolecular decomposition pathways of 2-oxepinoxy radical (1). The relative free energies (298 K, kcal/mol) at the B3LYP/ 6-311 + G(d,p)//B3LYP/6-31G(d) level are shown for each intemiediate relative to 1, and each free energy of activation is relative to the reactant for that specific step, [courtesy of Michael Fadden (J Phys Chem A 2000 104 8121-8130) Reprinted with permission of J Phys Chem A.]...

Fig. 12 Most favorable oxidative decomposition pathways for 2-oxepinoxy radical, at 298 K (path A) and 1250 K (path B). Adapted from Reference 131.

Even though 2-oxepinoxy radical (3 in Fig. 9b) may be an important intermediate in benzene oxidation, no confirmatory experimental evidence exists. Recently, Kroner et al. published an experimental study of the gas-phase acidity of 2(3H)-oxepinone (C6Hg02), obtained via flowing-afterglow They postulated that... 

A value of A/fadd = 352 2 kcal/mol was determined for Equation (37). This experimental evidence could ultimately be valuable in conclusively identifying 2-oxepinoxy radical as a reactive species of interest. 

The first of three experiments necessary to establish the heat of formation of 2-oxepinoxy radical 7 was the determination of gas-phase acidity of 2(3//)-oxepinone <2005JA7466>. The radical 7 is believed to be formed during the combustion of benzene, in particular, within internal combustion engines. The work is a contribution to the evaluation and substantiation of the two proposed pathways of the phenyl radical oxidation (see Scheme 1), the first (3—>4 —> 5 —> 7 —> 8) being thermodynamically and kinetically more accessible at T< 432 K whereas above 432 K, the latter (3 —> 4 —> 6 —> 8) is thermodynamically favored. 

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