A Computational Re-examination of the Criegee Intermediate–Sulfur Dioxide Reaction

The atmospheric oxidation of sulfur dioxide by the parent and dimethyl Criegee intermediates (CIs) may be an important source of sulfuric acid aerosol, which has a large impact on radiative forcing and therefore upon climate. A number of computational studies have considered how the CH₂OOS­(O)­O heteroozonide (HOZ) adduct formed in the CI + SO₂ reaction converts SO₂ to SO₃. In this work we use the CBS-QB3 quantum chemical method along with equation-of-motion spin-flip CCSD­(dT) and MCG3 theories to reveal new details regarding the formation and decomposition of the <i>endo</i> and <i>exo</i> conformers of the HOZ. Although ∼75% of the parent CI + SO₂ reaction is initiated by formation of the <i>exo</i> HOZ, hyperconjugation preferentially stabilizes many of the <i>endo</i> intermediates and transition structures by 1–5 kcal mol^–1. Our quantum chemical calculations, in conjunction with statistical rate theory models, predict a rate coefficient for the parent CI + SO₂ reaction of 3.68 × 10^–11 cm³ molecule^–1 s^–1, in good agreement with recent experimental measurements. RRKM/master equation simulations based on our quantum chemical data predict a prompt carbonyl + SO₃ yield of >95% for the reaction of both the parent and dimethyl CI with SO₂. The existence of concerted cycloreversion transition structures 10–15 kcal mol^–1 higher in energy than the HOZ accounts for most of the predicted SO₃ formation

Generation of Singlet Oxygen from Fragmentation of Monoactivated 1,1-Dihydroperoxides

The first singlet excited state of molecular oxygen (¹O₂) is an important oxidant in chemistry, biology, and medicine. ¹O₂ is most often generated through photosensitized excitation of ground-state oxygen. ¹O₂ can also be generated chemically through the decomposition of hydrogen peroxide and other peroxides. However, most of these “dark oxygenations” require water-rich media associated with short ¹O₂ lifetimes, and there is a need for oxygenations able to be conducted in organic solvents. We now report that monoactivated derivatives of 1,1-dihydroperoxides undergo a previously unobserved fragmentation to generate high yields of singlet molecular oxygen (¹O₂). The fragmentations, which can be conducted in a variety of organic solvents, require a geminal relationship between a peroxyanion and a peroxide activated toward heterolytic cleavage. The reaction is general for a range of skeletal frameworks and activating groups and, via in situ activation, can be applied directly to 1,1-dihydroperoxides. Our investigation suggests the fragmentation involves rate-limiting formation of a peroxyanion that decomposes via a Grob-like process

Generation of Singlet Oxygen from Fragmentation of Monoactivated 1,1-Dihydroperoxides

The first singlet excited state of molecular oxygen (¹O₂) is an important oxidant in chemistry, biology, and medicine. ¹O₂ is most often generated through photosensitized excitation of ground-state oxygen. ¹O₂ can also be generated chemically through the decomposition of hydrogen peroxide and other peroxides. However, most of these “dark oxygenations” require water-rich media associated with short ¹O₂ lifetimes, and there is a need for oxygenations able to be conducted in organic solvents. We now report that monoactivated derivatives of 1,1-dihydroperoxides undergo a previously unobserved fragmentation to generate high yields of singlet molecular oxygen (¹O₂). The fragmentations, which can be conducted in a variety of organic solvents, require a geminal relationship between a peroxyanion and a peroxide activated toward heterolytic cleavage. The reaction is general for a range of skeletal frameworks and activating groups and, via in situ activation, can be applied directly to 1,1-dihydroperoxides. Our investigation suggests the fragmentation involves rate-limiting formation of a peroxyanion that decomposes via a Grob-like process

Quantum Chemical and Statistical Rate Theory Studies of the Vinyl Hydroperoxides Formed in <i>trans</i>-2-Butene and 2,3-Dimethyl-2-butene Ozonolysis

The vinyl hydroperoxide (VHP), the major isomerization product of the syn-alkyl Criegee intermediate (CI) formed in alkene ozonolysis, is a direct precursor of hydroxyl radical (OH), the most important oxidant in the troposphere. While simulations of CI reactivity have usually assumed the VHP to be a prompt and quantitative source of OH, recent quantum chemical studies have revealed subtleties in VHP reactivity such as a barrier to peroxy bond homolysis and a possible rearrangement to a hydroxycarbonyl. In this work, we use M06-L, Weizmann-1 Brueckner Doubles, and equation-of-motion spin-flip coupled-cluster theories to calculate a comprehensive reaction mechanism for the syn and anti conformers of the parent VHP formed in <i>trans</i>-2-butene ozonolysis and the 1-methyl VHP formed in 2,3-dimethyl-2-butene ozonolysis. We predict that for the parent VHP the anti homolysis transition structure (TS) is 3 kcal mol^–1 lower in energy than the syn TS, but for the 1-methyl system, the syn TS is 2 kcal mol^–1 lower in energy. Statistical rate theory simulations based on the quantum chemical data predict that the parent VHP preferentially decomposes to vinoxy and OH radicals under all tropospheric conditions, while the 1-methyl VHP preferentially decomposes to 1-methylvinoxy and OH radicals only close to 298 K; at 200 K, the 1-methyl VHP preferentially rearranges to hydroxyacetone. Lower temperatures and higher pressures favor the temporary accumulation of both the parent and the 1-methyl VHP

Mechanism of the Intramolecular Hexadehydro-Diels–Alder Reaction

Theoretical analysis of the mechanism of the intramolecular hexadehydro-Diels–Alder (HDDA) reaction, validated against prior and newly measured kinetic data for a number of different tethered yne-diynes, indicates that the reaction proceeds in a highly asynchronous fashion. The rate-determining step is bond formation at the alkyne termini nearest the tether, which involves a transition-state structure exhibiting substantial diradical character. Whether the reaction then continues to close the remaining bond in a concerted fashion or in a stepwise fashion (i.e., with an intervening intermediate) depends on the substituents at the remaining terminal alkyne positions. Computational modeling of the HDDA reaction is complicated by the significant diradical character that arises along the reaction coordinate, which leads to instabilities in both restricted singlet Kohn–Sham density functional theory (DFT) and coupled cluster theory based on a Hartree–Fock reference wave function. A consistent picture emerges, however, from comparison of broken-symmetry DFT calculations and second-order perturbation theory based on complete-active-space self-consistent-field (CASPT2) calculations