Cl Atoms and OH Radicals Initiated Kinetic and Mechanistic Study on the Degradation of Propyl Butanoate under Tropospheric Conditions

The reactivity of various OVOCs (mainly esters) in the troposphere leads to the generation of various organics, which in turn leads to an increase in the cloud acidity of the Earth’s atmosphere. Hence, it becomes necessary to understand the mechanistic aspects of the reaction of these molecules with dominant atmospheric agents. In this study, the tropospheric degradation of one such ester, propyl butanoate (PB; CH₃CH₂CH₂COOCH₂CH₂CH₃) was studied with OH radicals and Cl atoms at the CCSD­(T)//M06-2x/6-311+G­(2d,2p) and CCSD­(T)//BHandHLYP/6-311+G­(2d,2p) level of theories over the studied temperature range of 200–400 K. The Arrhenius expressions obtained using the CVT/SCT/ISPE method were calculated as k_PB + Cl (200–400 K) = 1.3 × 10^–14 T^1.3 exp­[1335/T] cm³ molecule^–1 s^–1 and k_PB + OH (200–400 K) = 1.8 × 10^–26 T^4.6 exp­[4469/T] cm³ molecule^–1 s^–1. The obtained kinetics was also well validated against the SAR (structure–activity relationship)-based rate coefficients. The most prominent H-abstraction reaction channels were investigated for the PB + OH/Cl reaction. The abstraction of H atoms attached to the carbon atom present in the β-position to the ester (−C­(O)­O−) functionality was found to go via the lowest energy activation barriers for the reaction of PB toward both OH radicals and Cl atoms. The product degradation channels were also elucidated in an O₂/NO_x-rich environment. Moreover, to gauge the impact of the emitted PB on the troposphere, atmospheric lifetimes, radiative efficiencies, global warming potentials, and photochemical ozone creation potentials were also calculated and are included in the manuscript.