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A detailed computational study of the atmospheric reaction of the simplest Criegee intermediate CH 2 OO with methane has been performed using the density functional theory (DFT) method and high-level calculations. Solvation models were utilized to address the effect of water molecules on prominent reaction steps and their associated energies. The structures of all proposed mechanisms were optimized using B3LYP functional with several basis sets: 6-31G(d), 6-31G (2df,p), 6-311++G(3df,3pd) and at M06-2X/6-31G(d) and APFD/6-31G(d) levels of theory. Furthermore, all structures were optimized at the B3LYP/6-311++G(3df,3pd) level of theory. The intrinsic reaction coordinate (IRC) analysis was performed for characterizing the transition states on the potential energy surfaces. Fifteen different mechanistic pathways were studied for the reaction of Criegee intermediate with methane. Both thermodynamic functions (Δ H  and Δ G ), and activation parameters (activation energies E a , enthalpies of activation ΔH ǂ , and Gibbs energies of activation ΔG ǂ ) were calculated for all pathways investigated. The individual mechanisms for pathways  A1 ,  A2 ,  B1 , and  B2 , comprise two key steps: (i) the formation of ethyl hydroperoxide (EHP) accompanying with the hydrogen transfer from the alkanes to the terminal oxygen atom of CIs, and (ii) a following unimolecular dissociation of EHP. Pathways from  C1 → H1  involve the bimolecular reaction of EHP with different atmospheric species. The photochemical reaction of methane with EHP (pathway  E1 ) was found to be the most plausible reaction mechanism, exhibiting an overall activation energy of 7 kJ mol −1 , which was estimated in vacuum at the B3LYP/6-311++G(3df,3pd) level of theory. All of the reactions were found to be strongly exothermic, expect the case of the sulfur dioxide-involved pathway that is predicted to be endothermic. The solvent effect plays an important role in the reaction of EHP with ammonia (pathway  F1 ). Compared with the gas phase reaction, the overall activation energy for the solution phase reaction is decreased by 162 and 140 kJ mol −1  according to calculations done with the SMD and PCM solvation models, respectively.

Computational mechanistic study of the unimolecular dissociation of ethyl hydroperoxide and its bimolecular reactions with atmospheric species