The main purpose of this study is to assess the relative importance of diradical or peroxirane (perepoxide) intermediates in the singlet oxygen cycloaddition reactions with alkenes that lead to dioxetanes. The relevant nonconcerted pathways are explored for ethene, methyl vinyl ether, and s-trans-butadiene by CAS-MCSCF optimizations followed by multireference perturbative CAS-PT2 energy calculations and by DFT(B3LYP) optimizations. The two different theoretical approaches gave similar results (reported below). These results show that methoxy or vinyl substitution does not affect qualitatively the reaction features evidenced by the unsubstituted system. Peroxirane turns out to be attainable only by passing through the diradical, due to the nature of the critical points involved. The energy barriers for the transformation of the diradical to peroxirane in the case of ethene (ΔE ⧧ = 13−15 kcal mol-1) and methyl vinyl ether (ΔE ⧧ = 12−13 kcal mol-1) are higher than those for the diradical closure to dioxetane (ΔE ⧧ = 8−9 kcal mol-1, for ethene, and 9 kcal mol-1, for methyl vinyl ether). In all three systems, the peroxirane pathway to dioxetane is prevented by the high energy barrier for the second step, leading from peroxirane to dioxetane (ΔE ⧧ = 26−27, 27−31 and 22 kcal mol-1, for ethene, methyl vinyl ether, and butadiene, respectively). By contrast, peroxirane can very easily back-transform to the diradical (with a ΔE ⧧ estimate of 3 kcal mol-1, for ethene and methyl vinyl ether, and close to zero, for butadiene). These results indicate that, although a peroxirane intermediate might form in some cases, it corresponds to a dead-end pathway which cannot lead to dioxetane.

In the tropospheric oxidation of benzene and methylated benzenes, unsaturated dicarbonyls are commonly detected products. Aldehydes are known to contribute on their own to some aspects of air pollution, and hexa-2,4-dien-1,6-dial (muconaldehyde) in particular is interesting because of its multiform toxicity. This study investigates the likelyhood of some benzene oxidation steps and is especially focused on ring opening and generation of muconaldehyde. With sufficiently high NO x concentration, O abstraction by NO from the cis peroxyl group in the 2-hydroxy-cyclohexadienyl peroxyl radical III can play a role. In fact, it is shown to open a facile cascade of oxidation steps by first forming the 2-hydroxy-cyclohexadienyl oxyl radical VI. This intermediate is prone to ring opening via β-fragmentation and generates the open-chain delocalized 6-hydroxy-hexa-2,4-dienalyl radical VII, in which one terminus is the first carbonyl group of the final dialdehyde. The second one can form either by simple H abstraction operated by O2 or by O2 addition followed by HOO• elimination. The overall free-energy drop with respect to III is estimated to be 48 kcal mol-1. Exploration of other pathways, possibly playing a major role in yielding aldehydes in the case of low NO x concentration, indicates that only ring closure of the 2-hydroxy-cyclohexadienyl peroxyl radical III to the [3.2.1] bicyclic endo-peroxy allyl radical intermediate XIII is promising. In this case, however, the outcome of a subsequent ring opening can ultimately be the production of 1,2 and 1,4 dialdehydes (as direct oxidation of muconaldehyde itself can actually do).

No abstract

Master equation calculations were carried out to simulate the production of hydroxyl free radicals initiated by the reaction of acetyl free radicals (CH3(C=O).) with molecular oxygen. In particular, the competition between the unimolecular reactions and bimolecular reactions of vibrationally excited intermediates was modeled by using a single master equation. The vibrationally excited intermediates (isomers of acetylperoxyl radicals) result from the initial reaction of acetyl free radical with O2. The bimolecular reactions were modeled using a novel pseudo-first-order microcanonical rate constant approach. Stationary points on the multi-well, multi-channel potential energy surface (PES) were calculated at the DFT(B3LYP)/6-311G(2df,p) level of theory. Some additional calculations were carried out at the CASPT2(7,5)/6-31G(d) level of theory to investigate barrierless reactions and other features of the PES. The master equation simulations are in excellent agreement with the experimental OH yields measured in N2 or He buffer gas near 300 K, but they do not explain a recent report that the OH yields are independent of pressure in nearly pure O2 buffer gas.

Three different attacks of 3 Σ g O 2 on the hydroxycyclohexadienyl radical intermediate I (generated from the reaction of OH • with benzene) have been studied by Density Functional Theory. Both abstraction by O 2 of the hydrogen gem to OH in I (affording phenol) and O 2 addition to the π-delocalized system of I (producing a hydroxycyclohexadienyl peroxyl radical intermediate) appear to be very viable, with ∆H q ) 3-4 kcal mol -1 . The former reaction is exothermic by 27 kcal mol -1 , the latter only by 1 kcal mol -1 . In contrast, a recently repropounded pathway, which would lead to benzene oxide/oxepin, via hydrogen abstraction from the hydroxyl in I operated by O 2 , appears not to be competitive, showing a significantly higher barrier (∆H q ) 32 kcal mol -1 ). Benzene oxide and oxepin are estimated to lie 21 and 19 kcal mol -1 above I, respectively.