Degradation mechanism of propylene carbonate initiated by hydroxyl radical and fate of its product radicals: A hybrid density functional study

摘要

Degradation study of propylene carbonate (PC) with atmospheric oxidants are essential to understand its impact on the atmospheric environment and human health. Thus, we have analyzed the reaction mechanism and kinetics of PC with OH radical using quantum chemical study. In this regards, energies and frequencies calculations of all species involved in the possible oxidation channels (abstraction and addition) were performed using M06-2X functional along with 6-311 + + G (d,p) basis set. Our calculated energy results show that H-abstraction from -CH and -CH2 sites of PC are more dominant reaction channels compared to others. Further kinetic results also indicate that these two H-abstraction channels have a faster reaction rate. The calculated overall rate constant for the reaction was found to be 3.03 x 10(-12) cm(3) molecule(-1) s(-1) which is in good agreement with the experimental reported rate constant value (2.52 +/- 0.51) x 10(-12) cm(3) molecule(-1) s(-1). The contributions of percent branching ratios of these two channels (H-abstraction from -CH and -CH2 sites of PC) were found to be 66% and 27% respectively, while other reaction channels have very small contributions to the overall rate. The atmospheric lifetime of PC for the reaction with OH radicals was found to be 3.8 days. Further, the degradation mechanisms of product radicals were also considered in this work. During this investigation, we found that [1,3] dioxolane-2,4-dione, 5-methy-[1,3]dioxolane-2,4-dione, acetyl formyl carbonate (CH3C(O)OC(O)OCHO, CH3CHO, CH2O and CO2 are the end products. These result also suggest that the formation of CH3C(O)OC(O) OCHO is thermodynamically more favourable end product.