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

Abstract

3-Methyl-2-butene-1-thiol ((CH 3 ) 2 C�CH−CH 2 − SH; MBT) is a recently identified volatile organosulfur compound emitted from Cannabis sativa and is purported to contribute to its skunky odor. To understand its environmental fate, hydroxyl radical ( • OH)-mediated oxidation of MBT was conducted using high-level quantum chemical and theoretical kinetic calculations. Three stable conformers were identified for the title molecule. Abstraction and addition pathways are possible for the MBT + OH radical reaction, and thus, potential energy surfaces involving Habstraction and • OH addition were computed at the CCSD(T)/ aug-cc-pV(T+d)Z//M06-2X/aug-cc-pV(T+d)Z level of theory. The barrier height for the addition of the OH radical to a C atom of the alkene moiety, leading to the formation of a Ccentered MBT−OH radical, was computed to be −4.1 kcal mol −1 below the energy of the starting MBT + OH radical-separated reactants. This reaction was found to be dominant compared to other site-specific H-abstraction and addition paths. The kinetics of all the site-specific abstraction and addition reactions associated with the most stable MBT + OH radical reaction were assessed using the MESMER kinetic code between 200 and 320 K. Further, we considered the contributions from two other conformers of MBT to the overall reaction of MBT + OH radical. The estimated global rate coefficient for the oxidation of MBT with respect to its reactions with the OH radical was found to be 6.1 × 10 −11 cm 3 molecule −1 s −1 at 298 K and 1 atm pressure. The thermodynamic parameters and atmospheric implications of the MBT + OH reaction are discussed.

Investigating the Atmospheric Fate and Kinetics of OH Radical-Initiated Oxidation Reactions for Epoxybutane Isomers: Theoretical Insight

“…The contribution of each of these conformers at a particular temperature was accounted for by calculating the Boltzmann population distribution of these conformers. The Boltzmann population distribution in terms of Gibbs free energy change is given as p false( normalI false) = exp false( prefix− normalΔ G R T false) Q …”

“… k false( overall false) = k normalC ‐ normalI × p false( normalI false) + k normalC ‐ II × p false( II false) + k normalC ‐ III × p false( III false) where k C‑I , k C‑II , and k C‑III are the total rate coefficients obtained for the reactions of C-I + • OH, C-II + • OH, and C-III + • OH. A similar approach has been used previously. − …”

“…The overall rate coefficient considering all three conformers was calculated using eq where k C‑I , k C‑II , and k C‑III are the total rate coefficients obtained for the reactions of C-I + • OH, C-II + • OH, and C-III + • OH.…”

“… where k C‑I , k C‑II , and k C‑III are the total rate coefficients obtained for the reactions of C-I + • OH, C-II + • OH, and C-III + • OH. A similar approach has been used previously. − …”