# A Computational Study on the Atmospheric Fate of Carbon-Centered Radicals from the 3‑Methyl-2-butene-1-thiol + •OH Reaction: Mechanistic Insights and Atmospheric Implications

## TL;DR

This study explores how a specific sulfur-containing compound reacts in the atmosphere, forming reactive radicals that influence air chemistry.

## Contribution

The paper provides new mechanistic insights into the atmospheric transformation of MBT-derived radicals through computational analysis.

## Key findings

- R1 and R2 radicals from MBT + OH reaction form peroxy radicals that undergo rapid intramolecular hydrogen atom transfer.
- MBT-derived peroxy radicals contribute to tropospheric chemistry by generating reactive species like highly oxygenated radicals and alkyl radicals.

## Abstract

The reaction of 3-methyl-2-butene-1-thiol (MBT; (CH3)2CCHCH2SH) with the OH radical
is
reported to proceed via the addition to either of the sp2 hybridized C atoms, forming the two distinct C-centered radicals:
(CH3)2C­(OH)­C•HCH2SH (R1) and (CH3)2C•CH­(OH)­CH2SH (R2). Understanding the fate of these radicals is important
for elucidating MBT’s atmospheric transformation mechanisms
and the reaction products. Using quantum chemical calculations and
kinetic modeling, we show that the unimolecular dissociation as well
as isomerization reactions of R1 are kinetically unfavorable due to
high energy barriers, and that R1 most likely reacts with atmospheric
O2 to form R1O2 ((CH3)2C­(OH)­CH­(OO•)­CH2SH). In contrast, R2
can either undergo isomerization to form the sulfur-centered MBT–OH
radical or add O2 to form R2O2 ((CH3)2C­(OO•)­CH­(OH)­CH2SH). These
radicals undergo HO2 elimination and intramolecular hydrogen
atom transfer (HAT) pathways. Specifically, intramolecular HAT from
the –SH group to the terminal oxygen atom of R–OO forms
S-centered QOOH radicals, with barrier heights of −18.6 and
−18.3 kcal mol–1 for R1O2 and
R2O2, respectively, calculated relative to those of the
R1 + O2 and R2 + O2 reactants. Rate coefficients
for key pathways, including unimolecular dissociation and O2 addition followed by subsequent reactions, were calculated and analyzed.
The kinetics results suggest that the intramolecular H atom transfer
paths of R1O2 and R2O2 are significantly faster
by ∼3 orders of magnitude compared to their bimolecular reactions
with NO/HO2, respectively. The findings suggest that under
low NO concentrations R1O2 and R2O2 are capable
of undergoing H-shift-driven autoxidation mechanisms. The atmospheric
implications are discussed. Results indicate that MBT-derived peroxy
radicals contribute to tropospheric chemistry by generating reactive
species such as highly oxygenated peroxy radicals, HC­(O)­CH2SH, (CH3)2C­(OH)­C­(O)­H, CH3C­(O)­CH3, and various S- and C-centered alkyl radicals
in the atmosphere.