Multiple Stable Isoprene–Ozone Complexes Reveal Complex Entrance Channel Dynamics in the Isoprene + Ozone Reaction

Abstract

Accurately characterizing isoprene ozonolysis continues to challenge atmospheric chemists. The reaction is believed to be a spontaneous, concerted cycloaddition. However, little information is available about the entrance channel and isoprene–ozone complexes thought to define the long-range portion of the reaction coordinate. Our coupled cluster and auxiliary field quantum Monte Carlo calculations predict multiple stable isoprene–ozone van der Waals complexes for trans-isoprene in the gas phase with moderate association energies. These results indicate that long-range dynamics in the isoprene–ozone entrance channel can impact the overall reaction in the troposphere and provide the spectroscopic information necessary to extend the microwave characterization of isoprene ozonolysis to prereactive complexes. At the air–water interface, Born–Oppenheimer molecular dynamics simulations indicate that the cycloaddition reaction between ozone and trans-isoprene follows a stepwise mechanism, which is quite distinct from our proposed gas-phase mechanism and occurs on a femtosecond time scale. The stepwise nature of isoprene ozonolysis on the aqueous surface is more consistent with the DeMore mechanism than with the Criegee mechanism suggested by the gas-phase calculations, suggesting that the reaction media may play an important role in the reaction. Overall, these predictions aim to provide a missing fundamental piece of molecular insight into isoprene ozonolysis, which has broad tropospheric implications due to its critical role as a nighttime source of hydroxyl radicals.

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

“…Despite this consensus, two distinct proposals for the mechanism of primary ozonide formation have gained popularity: a concerted addition across the double bond (Criegee mechanism) and a stepwise addition, where ozone reacts with an olefin to form a biradical intermediate which may subsequently collapse to the ozonide (DeMore mechanism) . Research increasingly indicates that the Criegee mechanism prevails in simple (i.e., less substituted, electronically symmetric) systems, − while both steric and electronic effects of olefin substituents increase the activity of DeMore channels. ,, Recent theoretical work indicates that solvent effects can make stepwise mechanisms dominant in systems where they otherwise would not be …”

Structural Effects of Amines in Enhancing Methanesulfonic Acid-Driven New Particle Formation

“…29 In fact, the change from an H atom to a −CH 3 group leads to not only lower H-bond capacity but also higher steric hindrance in DMA compared to MA. 56,57 Because MSA also contains an additional −CH 3 group compared to SA, the role of steric hindrance of −CH 3 groups of DMA should be more pronounced when DMA interacts with MSA, especially in clusters with several DMA and MSA molecules. 56,57 However, to the best of our knowledge, no previous study has addressed how steric hindrance interplays with H-bond capacity and basicity in determining the enhancing potentials of amines on MSA-driven NPF.…”

Improved computational modeling of the kinetics of the acetylperoxy + HO2 reaction

“…These methods should provide more accurate energetics than CBS-QB3 for such species. 17,18 Second, we include all conformers of the minima and transition structures in our statistical rate theory calculations. Explicit treatment of all energetically relevant conformers can be critical for obtaining highly accurate predictions for peroxy radical reactions.…”