Oxidation reaction mechanism and kinetics between OH radicals and alkyl-substituted aliphatic thiols: H-abstraction pathways

Abstract

Kinetic rate constants for the oxidation reaction of the hydroxyl radical with CH3SH, C2H5SH, n-C3H7SH, and iso-C3H7SH under inert (Ar) conditions over the temperature range 252–430 K have been studied theoretically using density functional theory along with various exchange–correlation functionals as well as the benchmark CBS-QB3 quantum chemical approach. Bimolecular rate constants were estimated using transition state theory and the statistical Rice–Ramsperger–Kassel–Marcus theory. Comparison with experiment confirms that in the OH addition reaction pathways leading to the related products, the first bimolecular reaction steps have effective negative activation energy barriers. Effective rate constants have been calculated according to a steady-state analysis of a two-step model reaction mechanism. As a consequence of the negative activation energies, pressures higher than 104 bar are required to reach the high-pressure limit. Both from thermodynamic and kinetic viewpoints, the most favorable process here is the oxidation reaction of hydroxyl radicals with n-C3H7SH.

Theoretical Insights into the Gas-Phase Oxidation of 3-Methyl-2-butene-1-thiol by the OH Radical: Thermochemical and Kinetic Analysis

“…Therefore, more information on the atmospheric transformation mechanism of this compound in the troposphere is required to assess its potential impact. Once released, MBT is expected to encounter and interact with hydroxyl (OH) radicals, as OH radicals are the most prominent reactive intermediates in atmospheric chemistry. , In addition, previous studies have indicated that OH radicals play a dominant role in the oxidation of aliphatic thiols under tropospheric conditions. ,, …”

“…While there are several reports on the fate of saturated aliphatic thiols in the presence of OH radicals under atmospheric conditions, there are no data available on the oxidation of plant-derived unsaturated thiols initiated by OH radicals. Several experimental and theoretical studies have been devoted to the mechanism and products formed from the OH radical-mediated oxidation of saturated aliphatic thiols. ,− For example, the kinetics of OH radical reactions with aliphatic thiols have been studied by Barnes et al, Lee and Tang, and Wine et al The results from these studies suggest that thiols react with OH radicals at nearly the same rate and mainly occur through H atom abstraction by OH radicals from the −SH group. ,, In addition, a theoretical study of the gas phase reaction mechanism of C1–C3 aliphatic thiols with OH radicals has been reported, and the results suggested that H atom abstraction from the −SH group is the major contributor to the overall reaction compared to H atom abstraction from the alkyl groups. The rate coefficients for the reaction of CH 3 SH + OH radical, C 2 H 5 SH + OH radical, and n- C 3 H 7 SH + OH radical at 298 K are reported to be 1.3 × 10 –11 , 1.5 × 10 –11 , and 3.0 × 10 –11 cm 3 molecule –1 s –1 , respectively.…”

“…16,17 In addition, previous studies have indicated that OH radicals play a dominant role in the oxidation of aliphatic thiols under tropospheric conditions. 3,18,19 While there are several reports on the fate of saturated aliphatic thiols in the presence of OH radicals under atmospheric conditions, there are no data available on the oxidation of plant-derived unsaturated thiols initiated by OH radicals. Several experimental and theoretical studies have been devoted to the mechanism and products formed from the OH radical-mediated oxidation of saturated aliphatic thiols.…”

“…16 , 17 In addition, previous studies have indicated that OH radicals play a dominant role in the oxidation of aliphatic thiols under tropospheric conditions. 3 , 18 , 19 …”

“…Several experimental and theoretical studies have been devoted to the mechanism and products formed from the OH radical-mediated oxidation of saturated aliphatic thiols. 3 , 19 − 22 For example, the kinetics of OH radical reactions with aliphatic thiols have been studied by Barnes et al, 22 Lee and Tang, 20 and Wine et al 3 The results from these studies suggest that thiols react with OH radicals at nearly the same rate and mainly occur through H atom abstraction by OH radicals from the −SH group. 3 , 20 , 22 In addition, a theoretical study of the gas phase reaction mechanism of C1–C3 aliphatic thiols with OH radicals has been reported, 18 and the results suggested that H atom abstraction from the −SH group is the major contributor to the overall reaction compared to H atom abstraction from the alkyl groups.…”