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

Mansour H Almatarneh,Mohammad A Halim,Yuming Zhao,Mohammednoor Altarawneh,Asmaa Alnajajrah

doi:10.1038/s41598-020-71881-3

Abstract

A detailed computational study of the atmospheric reaction of the simplest Criegee intermediate CH2OO 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 (ΔH and ΔG), and activation parameters (activation energies Ea, enthalpies of activation ΔHǂ, and Gibbs energies of activation ΔGǂ) 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.

Highlights

A detailed computational study of the atmospheric reaction of the simplest Criegee intermediate CH2OO with methane has been performed using the density functional theory (DFT) method and high-level calculations
Comprehensive quantum chemistry calculations of fifteen possible reaction pathways were conducted for unimolecular dissociation and bimolecular reactions of ethyl hydroperoxide (EHP)
TSG2 shows a C–Cl bond in chloromethane elongated from 1.80 to 2.60 Å to form Cl and methyl radical, which are in great concurrence with the work reported by Evanseck et al, This study investigates that the geometry of the transition state shows an extension of the C–Cl bond length to 2.10 from 1.79 Å in methyl c hloride[47], which is accompanied with a slight increase in the bond length of O–H bond length by 0.110 Å to 1.00 Å

Summary

Introduction

A detailed computational study of the atmospheric reaction of the simplest Criegee intermediate CH2OO with methane has been performed using the density functional theory (DFT) method and high-level calculations. The title reaction influences other fundamentally important phenomena in the atmosphere, namely, solar radiation, temperature gradients, and air dynamics They are expected to play an important role in processes determining the interactions between the atmosphere and the biosphere. Methane assumes a significant role in the global carbon cycle and energy utilization and exerts important effects on atmospheric chemistry and climate. CIs are significant intermediates in the environment, where they play a major role in the formation of hydroxyl radicals and different organic compounds. The initial step in the ozonolysis of alkenes is the 1,3-dipolar cycloaddition reaction of ozone to the C=C double bond, which gives a 5-membered ring intermediate called a primary ozonide (POZ). This step is followed by a rapid decomposition reaction (see Fig. 1), in which the O–O and C–C bonds of POZ are cleaved together to yield a CI and a ketone product[20,21]

Methods

Results

Conclusion