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.

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.

Alkyl peroxyls form in the atmospheric oxidation of hydrocarbons and in their combustion. When NO concentration is low, they can appreciably react with themselves. This reaction has both propagation and termination channels. Multireference second-order perturbative energy calculations CAS(16,12)-PT2/6-311G(2df,p) have been carried out on the CAS(8,8)-MCSCF/6-311G(d,p) geometries pertaining to the reaction pathways explored. The tetroxide intermediate put forward first by Russell in 1957 is found as a stable energy minimum, but the calculations indicate that, as the system moves from atmospheric to combustion temperatures, its formation becomes problematic. A concerted synchronous transition structure, apt to connect it with the termination products, formaldehyde, methanol, and dioxygen, is not found. The concerted dissociation of the two external O–O bonds in the tetroxide leads to the (CH23O•)23⋯3O2 complex, with overall singlet spin multiplicity. Both termination via H transfer, to give H2CO, CH3OH, and O2, or dissociation to 2 CH3O•+O2 (possible propagation) are feasible. The former could occur in principle with production of either excited O21 or excited H23CO. However, if a sufficiently easy intersystem crossing (ISC) could take place in the complex, the process would end up with all ground-state molecules. The (possible) propagation channels are favored by higher temperatures, while lower temperatures favor the ISC mediated termination channel. A fairly good qualitative agreement with experimental T dependence of the relevant branching ratio is found. From the tetroxide over again, dissociation of a single external O–O bond leads to CH3O• and CH3O3•, or possibly to a (CH3O•⋯CH3O3•)1 complex, but further transformations along this line are not competitive.

The [1,2] and [2,3] migration steps in the Stevens and Sommelet-Hauser rearrangements which occur in the ylides of quaternary ammonium salts have been studied at M05-2x levels. The Stevens migration has been found to take place through a diradical pathway in several cases (tetramethylammonium, benzyltrimethylammonium, benzylphenacyldimethylammonium ylides). By contrast, in the phenyltrimethylammonium ylide this reaction takes place through a concerted process. The Sommelet-Hauser rearrangement takes place through a concerted transition structure. The most important factor determining the extent of competition with the Stevens rearrangement is the difference in the reaction energies as the formation of the Sommelet-Hauser intermediate is significantly less endoergic.

The mechanisms of the fragmentation and isomerization pathways of o-benzyne were studied at the multi-configurational second-order perturbative level [CAS(12,12)-PT2]. The direct fragmentation of o-benzyne to C2H2 + C4H2 follows two mechanisms: a concerted mechanism and a stepwise mechanism. Although the concerted mechanism is characterized by a single closed-shell transition structure, the stepwise pathway is more complex and structures with a strong diradical character are seen. A third diradicaloid fragmentation pathway of o-benzyne yields C6H2 as the final product. As an alternative to fragmentation, o-benzyne can also undergo rearrangement to its meta and para isomers and to the open chain cis and trans isomers of hexa-3-en-1,6-diyne (HED). These easily fragment to C2H2 + C4H2 or C6H2. Kinetic modelling at several different temperatures between 800 and 3000 K predicted that the thermal decomposition of o-benzyne should yield C2H2, C4H2 and C6H2 as the main products. Small amounts of the HED isomers accumulated at temperatures <1200 K, but they rapidly decompose at higher temperatures. Between 1000 and 1400 K, C2H2 + C4H2 are formed exclusively from the decomposition of trans-HED. At temperatures >1400 K, C2H2 + C4H2 also form from the direct fragmentation of o-benzyne. The formation of C2H2 + C4H2 prevails up to 1600 K but above this temperature the formation of C6H2 prevails. At temperatures >2400 K, the direct fragmentation of o-benzyne again leads to the formation of C2H2 + C4H2. The formation of hydrogen atoms is also explained by our proposed mechanisms.