Kinetic and mechanistic insights into the atmospheric hydrogen abstraction of ۳-Hydroxybutanal by chlorine

This work investigates the reaction mechanism of hydrogen atom abstraction from ۳-hydroxybutanal (۳HB) by the chlorine (Cl) radical under atmospheric conditions, a process relevant to the degradation of volatile organic compounds (VOCs). Density Functional Theory (DFT) calculations were carried out at the B۳LYP/def۲-SVP level to explore five possible abstraction pathways, each involving a distinct hydrogen site within the ۳HB molecule. Vibrational frequency analyses confirmed the presence of five corresponding transition states (TS۱–TS۵). Rate constants for each pathway were computed using KiSThelP software across a temperature range of ۲۷۸–۴۰۰ K. The calculated rate constants are ۲.۵۸۱۷ × ۱۰¹, ۱.۶۶۹۵ × ۱۰-۷, ۸.۳۹۷۵ × ۱۰², ۱.۱۷۵۸ × ۱۰-۱۱, and ۲.۳۰۵۶ × ۱۰-۱۰ cm³•molecule-۱•s-۱ for pathways ۱ through ۵, respectively. These results indicate that hydrogen abstraction at site ۳ (pathway ۳) is the most kinetically favorable. Thermodynamic analysis supports this conclusion, with the product from pathway ۳ exhibiting the lowest Gibbs free energy and reaction enthalpy (–۲۲.۲۵ kcal/mol), confirming it as the most thermodynamically stable. All pathways are exothermic and exergonic. These findings provide detailed mechanistic insight into the atmospheric reactivity of ۳HB and may be extended to other structurally related VOCs, contributing to a better understanding of atmospheric oxidation processes.

کلیدواژه ها:

Atmospheric reaction ، Reaction pathway ، Vibrational frequencies ، Transition State ، Kinetics and Thermodynamic parameters