H-Abstraction from Dimethyl Sulfide in the Presence of an Excess of Hydroxyl Radicals. A Quantum Chemical Evaluation of Thermochemical and Kinetic Parameters Unveils an Alternative Pathway to Dimethyl Sulfoxide

Abstract

Elucidation of the oxidation mechanism of naturally emitted reduced sulfur compounds, especially dimethyl sulfide, plays a central role in understanding background acid precipitation in the natural environment. Most frequently, theoretical studies of the addition and H-elimination reactions of dimethyl sulfide with hydroxyl radicals are studied considering the presence of oxygen that further reacts with the radicals formed in the initial steps. Although the reaction of intermediate species with additional hydroxyl radicals has been considered as part of the global mechanism of oxidation, few if any attention has been dedicated to the possibility of reactions of the initial radicals with a second • OH molecule. In this work we performed a computational study using quantum-chemical methods, of the mechanism of H-abstraction from dimethyl sulfide under normal atmospheric conditions and in reaction chambers at different O2 partial pressure, including complete absence of oxygen. Additionally, important rate coefficients were computed using canonical and variational transition state theory. The rate coefficient for abstraction affords a 4.72 x 10-12 cm 3 molecule-1 s-1 value, very close to the most recent experimental one (4.13 x 10-12 cm 3 molecule-1 s-1). According to our best results, the initial methyl thiomethyl radical was obtained at-25.2 kcal/mol (experimentally-22.4 kcal/mol), and four important paths were identified on the potential energy surface. From the interplay of thermochemical and kinetic arguments, it was possible to demonstrate that the preferred product of the reaction of dimethyl sulfide with two hydroxyl radicals, is actually dimethyl sulfoxide.

Reinvestigation of the Deceptively Simple Reaction of Toluene with OH and the Fate of the Benzyl Radical: The “Hidden” Routes to Cresols and Benzaldehyde

“…If OH is present in excess, then the route to the o , m -dihydroxy toluene (M16_oC) is favored. We had already shown the modifications that occur in the mechanism of oxidation of dimethyl sulfide (DMS) when excess OH is present; 48 , 49 this is a similar case, but with another addition to the ring instead of abstraction. If that is not the case, then the reaction with O 2 is favored, either by attack to the hydrogen ipso to the OH (M04_oC) (later passing through the transition state TS03_oC with a small barrier giving the cresol) or through bonding with the carbon bearing the lone electron in the ring (M07_oC).…”

Dipolar 1,3‐cycloaddition of thioformaldehydeS‐methylide (CH2SCH2) to ethylene and acetylene. A comparison with (valence) isoelectronicO3,SO2,CH2OOandCH2SO

“…The parent compound of the thiocarbonyl ylide family, thioformaldehyde S ‐methylide (TSM), was initially recognized by Knott in 1955 18 and later studied by several research groups using different spectroscopic and theoretical methods 11–15,19,20 . However, after the work by Sustmann et al 20 in 2003 concerning the mechanism of cycloaddition of TSM, the only available theoretical studies are those performed in the last 2 years by our groups 21–23 . In particular, we started by proposing an original path leading to TSM based on a detailed computational study of the H‐abstraction path in the oxidation of dimethyl sulfide (DMS) by OH radicals, under the hypothesis of high OH concentration 21,22 .…”

“…[11][12][13][14][15]19,20 However, after the work by Sustmann et al 20 in 2003 concerning the mechanism of cycloaddition of TSM, the only available theoretical studies are those performed in the last 2 years by our groups. [21][22][23] In particular, we started by proposing an original path leading to TSM based on a detailed computational study of the H-abstraction path in the oxidation of dimethyl sulfide (DMS) by OH radicals, under the hypothesis of high OH concentration. 21,22 We identified a reaction channel that could be open for the transformation of DMS into a closed shell planar structure where two methylene groups are bound to the central sulfur, namely TSM.…”

“…[21][22][23] In particular, we started by proposing an original path leading to TSM based on a detailed computational study of the H-abstraction path in the oxidation of dimethyl sulfide (DMS) by OH radicals, under the hypothesis of high OH concentration. 21,22 We identified a reaction channel that could be open for the transformation of DMS into a closed shell planar structure where two methylene groups are bound to the central sulfur, namely TSM. Next, we discussed the structural details in the context of the stationary points ruling the [SC 2 H 4 ] potential energy surface (PES).…”

Spectroscopic characterization of two peroxyl radicals during the O2-oxidation of the methylthio radical

“…On the other hand, the atmospheric oxidation of DMS can also proceed through the OH-addition pathway (Fig. 1), resulting the stepwise formation of additional VOSCs dimethyl sulfoxide (DMSO, CH 3 S(O)CH 3 ), and methanesulfinic acid (CH 3 S(O) OH) [15][16][17][18][19][20][21][22] .…”