Variational Transition-State Theory Rate Constant Calculations of the OH + CH₃SH Reaction and Several Isotopic Variants

Abstract

In this paper the first variational transition-state theory rate constant calculation for the OH + CH 3 SH reaction and several isotopic variants involving OD, CH 3 SD, and CD 3 SH is presented. Multidimensional tunneling corrections have been included when necessary. The potential energy surface has been described by lowlevel calculations at the MP2(full)/cc-pVDZ level combined with higher level calculations using the multilevel MCCM-CCSD(T)-CO-2m method. We have found that the reaction takes place by forming first a weakly bound complex that presents two hydrogen bonds between the hydroxyl radical and methanethiol. From this complex two different reaction pathways have been found: abstraction of the hydrogen atom attached to the sulfur atom and abstraction of the hydrogen atom of the methyl group. To take into account the different kinetic channels, the canonical and the competitive canonical unified statistical theories have been used to calculate the global rate constant of the perprotio reaction. Due to the slower nature of the H-abstraction of the methyl group, the global rate constant turns out to be equivalent to the overall rate constant for the abstraction of the hydrogen atom attached to the sulfur atom. It is only at the higher temperature range when a significant percentage of CH 2 SH is predicted. An activation energy of -0.54 kcal/mol in the range 225-430 K is obtained for the global reaction, in very good agreement with the experimental results. Several kinetic isotope effects of the global reactions have also been calculated and analyzed in view of previously published experimental results.

Kinetics of hydrogen activation on ceria–zirconia

“…In addition, the absence of such limitations was explicitly verified (vide supra). Instead, it is suggested that two types of active sites contribute to the observed reactivity [54]. Specifically, two types of active sites contribute to the observed reactivity at high temperature, whereas at low temperature only one type of the sites shows measurable activity.…”

Theoretical calculations of structures, energetics, and kinetics of O (3P) + CH3OH reactions

“…Note that the B3LYP and MPWKCIS1K methods underestimate the values for this torsion mode compared with the other DFT results. As discussed by Masgrau et al [38], the lower vibrational frequencies, mainly for the transitional modes (those vibrational modes at the transition state structure that come from translational or rotational modes at the reactants) dominate the computation of the rate constant and determine the curvature of the Arrhenius plots. Table III gives the calculated electronic barrier heights, electronic reaction energy, and enthalpy of reaction at 0 K for the three reactive mechanisms of the O ( [7,8] and previous theoretical results [5] are also included.…”

Insight into the reaction mechanism of CH2SH with HO2: A density functional theory study

“…The radical CH 3 S and its isomer CH 2 SH are key intermediates in atmospheric chemistry, which can be produced by the oxidation of organic sulfur or the combustion of coals containing high quantity of sulfur . As typical sulfur‐containing radicals, they may lead to many adverse effects such as causing acid rain, reducing air visibility, and causing global climate change . The researchers mainly focus on two aspects of CH 3 S and CH 2 SH: (a) the properties of special species, including thermochemistry and spectrum; (b) the reaction mechanisms and kinetics involved …”

“…[1] As typical sulfur-containing radicals, they may lead to many adverse effects such as causing acid rain, reducing air visibility, and causing global climate change. [2] The researchers mainly focus on two aspects of CH 3 S and CH 2 SH:(a) the properties of special species, including thermochemistry and spectrum; (b) the reaction mechanisms and kinetics involved. [1,3] CH 2 SH radical is reported to be more stable than CH 3 S in high-temperature region, while the latter is more stable in low-temperature region.…”

“…[1] As typical sulfur-containing radicals, they may lead to many adverse effects such as causing acid rain, reducing air visibility, and causing global climate change. [2] The researchers mainly focus on two aspects of CH 3 S and CH 2 SH:…”