Variational transition‐state theory study of the rate constant of the DMS·OH scavenging reaction by O₂

Abstract

The chemical tropospheric dimethyl sulfide (DMS, CH3SCH3) degradation involves several steps highly dependent on the environmental conditions. So, intensive efforts have been devoted during the last years to enhance the understanding of the DMS oxidation mechanism under different conditions. The reaction of DMS with OH is considered to be the most relevant process that initiates the whole oxidation process. The experimental observations have been explained by a two-channel mechanism consisting of a H-abstraction process leading to CH3S(O)CH3 and HO2 and an addition reaction leading to the DMS·OH adduct. In the presence of O2, the DMS·OH adduct is competitively scavenged increasing the contribution of the addition channel to the overall DMS oxidation. Recent experimental measurements have determined from a global fit that the rate constant of this scavenging process is independent of pressure and temperature but this rate constant cannot be directly measured. In this article, a variational transition-state theory calculation of the low- and high-pressure rate constants for the reaction between DMS·OH and O2 has been carried out as a function of temperature. Our proposal is that the slight temperature dependence of the scavenging rate constant can only be explained if the H-abstraction bottleneck is preceded by a dynamical bottleneck corresponding to the association process between the DMS·OH adduct and the O2 molecule. The agreement between the low-pressure and high-pressure rate constants confirms the experimental observations.

Formation pathways of DMSO2 in the addition channel of the OH‐initiated DMS oxidation: A theoretical study

“…Tables 1-7 present the classical potential energies and the adiabatic potential energies (that is, including the zero point energy corrections) with respect to the corresponding reactants for the stationary points of the different reaction pathways. As already observed in our previous study 5 the structures of the stationary points optimized on the MPW1K/6-311G(d,p) PES are very similar to the geometries at the MPW1K/MG3S level (see Fig. 1 and the Supp.…”

“…This onestep mechanism resembles the reaction of CH 3 S(O 2 )(OH)CH 3 with NO 2 and contrasts with the multi-step process of CH 3 S(OH)CH 3 with NO 2 , both studied in our previous work. 5 The presence of the O atom and the O 2 group bonded to sulfur atom in CH 3 S(O)(OH)CH 3 and CH 3 S(O 2 )(OH)CH 3 , respectively, causes certain steric hindrance in the approach of the NO 2 molecule to those radicals that may impede the formation of a SÁÁÁO interaction, thus leading to the direct abstraction of the hydrogen atom from the OH group by NO 2 . The free energy values at the stationary points presented in Figure 3 demonstrate that R2 is a thermodynamically allowed reaction pathway that should also present a high rate constant.…”

“…In our previous theoretical study 5 ab initio and density functional electronic structure calculations of all the possible reaction pathways corresponding to the reaction process initiated by DMS-OH 1 O 2 , leading to the formation of DMSO product in the presence of NO x (NO and NO 2 ), were carried out for the first time. Our theoretical results clearly showed the existence of NO x -dependent pathways leading to the formation of DMSO in addition to O 2 -dependent channels.…”

“…1 of ref. 5 ) 1 OH. The PCabs1 adduct appeared as a consequence of the H-abstraction reaction between DMSOH and O 2 as it was shown in our previous work.…”

“…Channel R6b corresponds to the reaction between CH 3 S(O 2 )(OH)CH 3 and NO that finally leads to the two pairs of products DMSO 1 HOONO and DMSO 2 1 HNO 2 . Note that the DMSO formation process corresponding to the R6b channel was already studied in our previous work 5 but we have included it here for completeness. The energetics corresponding to this channel is given in Table 7 and the free energy profile in Figure 7.…”