Multireference Study of the H2COO (Criegee Intermediate) + O3 Addition: A Reaction of Possible Tropospheric Interest

A comparable DFT study on reaction of CHCl•- with O3 and S2O

CONTEXT

In this discussion, we began building two model, S₂O + CHCl^•- and O₃ + CHCl^•-, using DFT-BHandHLYP method, to study their reactions mechanisms on singlet PES. For this purpose, we hope to explore the effects of the difference between sulfur and oxygen atoms on the CHCl^•- anion. Experimentalists and computer scientists may utilize the collected data to generate a wide range of hypotheses for experimental phenomena and predictions, allowing them to realize their full potential.

METHODS

The ion-molecule reaction mechanism of CHCl^•- with S₂O and O₃ was studied using the DFT-BHandHLYP level of theory with the aug-cc-pVDZ basis set. Our theoretical findings show that Path 6 is the favored reaction pathway for CHCl^•- + O₃ reaction as identified by the O-abstraction reaction pattern. Comparing to the direct H- and Cl-abstraction mechanisms, the reaction (CHCl^•- + S₂O) prefers the intramolecular S_N2 reaction pattern. Moreover, the calculated results demonstrated that the CHCl^•- + S₂O reaction is thermodynamically more favorable than the CHCl^•- + O₃ reaction, which is kinetically more advantageous. As a result, if the required reaction condition in the atmospheric process is met, the O₃ reaction will happen more effectively. In terms of kinetics and thermodynamics viewpoints, the CHCl^•- anion was very effective in eliminating S₂O and O₃.

Benchmarking CASPT3 Vertical Excitation Energies

Based on 280 reference vertical transition energies of various natures (singlet, triplet, valence, Rydberg, n → π∗, π → π∗, and double excitations) extracted from the QUEST database, we assess the accuracy of third-order multireference perturbation theory, CASPT3, in the context of molecular excited states. When one applies the disputable ionization- potential-electron-affinity (IPEA) shift, we show that CASPT3 provides a similar accuracy as its second-order counterpart, CASPT2, with the same mean absolute error of 0.11 eV. However, as already reported, we also observe that the accuracy of CASPT3 is almost insensitive to the IPEA shift, irrespective of the transition type and system size, with a small reduction of the mean absolute error to 0.09 eV when the IPEA shift is switched off.

Kinetics of the Gas Phase Reactions of the Criegee Intermediate CH2OO with O3 and IO

The kinetics of the gas phase reactions of the Criegee intermediate CH₂OO with O₃ and IO have been studied at 296 K and 300 Torr through simultaneous measurements of CH₂OO, the CH₂OO precursor (CH₂I₂), O₃, and IO using flash photolysis of CH₂I₂/O₂/O₃/N₂ mixtures at 248 nm coupled to time-resolved broadband UV absorption spectroscopy. Experiments were performed under pseudo-first-order conditions with respect to O₃, with the rate coefficients for reactions of CH₂OO with O₃ and IO obtained by fitting to the observed decays of CH₂OO using a model constrained to the measured concentrations of IO. Fits were performed globally, with the ratio between the initial concentration of O₃ and the average concentration of IO varying in the range 30-700, and gave k_CH2OO+O3 = (3.6 ± 0.8) × 10^-13 cm³ molecule^-1 s^-1 and k_CH2OO+IO = (7.6 ± 1.4) × 10^-11 cm³ molecule^-1 s^-1 (where the errors are at the 2σ level). The magnitude of k_CH2OO+O3 has a significant effect on the steady state concentration of CH₂OO in chamber studies. Atmospheric implications of the results are discussed.