Concerted and Stepwise Reaction Mechanisms for the Addition of Ozone to Acetylene: A Computational Study

Computational Studies of Rubber Ozonation Explain the Effectiveness of 6PPD as an Antidegradant and the Mechanism of Its Quinone Formation

The discovery that the commercial rubber antidegradant 6PPD reacts with ozone (O₃) to produce a highly toxic quinone (6PPDQ) spurred a significant research effort into nontoxic alternatives. This work has been hampered by lack of a detailed understanding of the mechanism of protection that 6PPD affords rubber compounds against ozone. Herein, we report high-level density functional theory studies into early steps of rubber and PPD (p-phenylenediamine) ozonation, identifying key steps that contribute to the antiozonant activity of PPDs. In this, we establish that our density functional theory approach can achieve chemical accuracy for many ozonation reactions, which are notoriously difficult to model. Using adiabatic energy decomposition analysis, we examine and dispel the notion that one-electron charge transfer initiates ozonation in these systems, as is sometimes argued. Instead, we find direct interaction between O₃ and the PPD aromatic ring is kinetically accessible and that this motif is more significant than interactions with PPD nitrogens. The former pathway results in a hydroxylated PPD intermediate, which reacts further with O₃ to afford 6PPD hydroquinone and, ultimately, 6PPDQ. This mechanism directly links the toxicity of 6PPDQ to the antiozonant function of 6PPD. These results have significant implications for development of alternative antiozonants, which are discussed.

Quantum modeling of the reaction between ozone and hydrogen cyanide

This work consists of an investigation, using current methods of quantum chemistry and, at first, on the basis of the available experimental results, about the new mechanisms of the reaction between ozone and hydrogen cyanide (HCN) in gaseous phases. Three possible reaction pathways which we have determined as the most probable and, all three, leading exactly to the same products, are proposed here. For each of these pathways, several steps for which we performed a kinetic study were identified in the singlet potential energy surface. To confirm the proposed mechanisms, we have achieved a study including the intrinsic reaction coordinate (IRC), the topological analysis of atoms in molecule and the harmonic vibrational frequencies calculations. The obtained results reveal that the final products have considerable thermodynamic stability and this reaction is exothermic in standard conditions.

Effect of Steric Congestion on the Stepwise Character and Synchronicity of a 1,3-Dipolar Reaction of a Nitrile Ylide and an Olefin

It has been hypothesised that steric congestion could play a key role in directing the 1,3-dipolar reaction mechanism in its shift from a concerted to a stepwise model. An example is given of the influence of steric congestion on the concertedness of a 1,3-dipolar reaction mechanism. Keeping other influences untouched, including the effects of electron-withdrawing or electron-donating groups, the nature of the dipole or dipolarophile, solvent, as well as the catalytic effects which have been kept fixed, this study focuses on the steric effect. At least in the case of the 1,3-dipolar cycloaddition of nitrile ylide and common olefins, when steric congestion serves as the only influence, it is unable to change the reaction mechanism from concerted into stepwise.

Theoretical investigations toward the tandem reactions of N-aziridinyl imine compounds forming triquinanes via trimethylenemethane diyls: mechanisms and stereoselectivity

In this paper, we have investigated the tandem reaction mechanism for the N-aziridinyl imine compounds forming triquinanes via trimethylenemethane (TMM) diyls in detail. Based on the calculated results, the reaction is initiated by the cleavage of the N-aziridinyl in the substrate, followed by an intramolecular 1,3-dipolar (3 + 2) cycloaddition preferentially leading to a linearly-fused tetrahydrocyclopentapyrazole intermediate. Next, the intermediate loses N2 to form the singlet TMM diyl M3S, which can then undergo another concerted (3 + 2) cycloaddition to generate the linearly-fused cis–trans or cis–syn triquinane products. In addition, M3S can also undergo intersystem crossing to the triplet TMM diyl M3T, and the six possible reaction pathways associated with M3T have also been identified. The calculated results reveal that the cis–trans fused pathway associated with M3S is energetically preferred with the highest free energy barrier of 25.0 kcal mol(−1). In comparison, the cyclization of M3T requires much higher activation free energies (ΔG(≠) = 34.4–57.8 kcal mol(−1)). At the experimental temperature 110 °C, only the linearly-fused cis–trans and cis–syn pathways associated with M3T (ΔG(≠) = 34.4 and 35.5 kcal mol(−1) respectively) are possible. The calculated results also indicate that for both M3S and M3T, the linearly-fused cis–trans triquinane should be the main product, which is consistent with the experimental observation. At last, conformational and NBO analyses on key transition states identified the cis–trans stereocontrol factors. Further calculations indicate that the methyl substituent on the allene group of the reactant substrate improves the stereoselectivity of the reaction but does not affect the rate-determining step.

Prediction of Mechanism and Thermochemical Properties of O3+ H2S Atmospheric Reaction

Ozone and hydrogen sulfide reaction mechanism including a complex was studied at the B3LYP/6-311++G(3df,3pd) and CCSD/6-311++G(3df,3pd)//B3LYP/6-311++G(3df,3pd) levels of computation. The interaction between sulfur atom of hydrogen sulfide and terminal oxygen atom of ozone produces a stable H2S-O3complex with no barrier. With the decomposition of this complex, four possible product channels have been found. Intrinsic reaction coordinate, topological analyses of atom in molecule, and vibrational frequency calculation have been used to confirm the suggested mechanism. Thermodynamic data atT= 298.15 K and the atmospheric pressure have been calculated. The results show that the production of H2O + SO2is the main reaction channel with ΔG° = −645.84 kJ/mol. Rate constants of H2S + O3reaction show two product channels, SO2 + H2O and HSO + HOO, which compete with each other based on the temperature.

Mechanism and stereoselectivity of the stepwise 1,3-dipolar cycloadditions between a thiocarbonyl ylide and electron-deficient dipolarophiles: a computational investigation

The 1,3-dipolar cycloadditions of a thiocarbonyl ylide to dimethyl 2,3-dicyanofumarate and 2,3-dicyanomaleate have been studied with DFT computations. The first C-C bond is formed via an anti attack to produce a very polar, zwitterionic diradical. The low stereoselectivity of the reaction was found to arise from rotations about single bonds in the intermediates that compete with cyclization. A distortion-interaction energy analysis was performed to compare the stepwise and concerted mechanisms, and to explain why the stepwise mechanism is favored in this unusual case.