Computational exploration of regioselectivity and atmospheric lifetime in NO3-initiated reactions of CH3OCH3 and CH3OCH2CH3

Theoretical investigation of the mechanism, kinetics and subsequent degradation products of the NO3 radical initiated oxidation of 4-hydroxy-3-hexanone

The oxidation mechanism of 4-hydroxy-3-hexanone (CH3CH2C(O)CH(OH)CH2CH3) initiated by NO3 radicals in the nighttime is investigated systematically by applying quantum theoretical methods. According to thermodynamic research, the process of H-abstraction on the -CH- group adjacent to the hydroxyl group is the most dominant pathway with the lowest activation energy. The analysis of Mulliken charge charts and molecular electrostatic potential maps illustrate that C-H bonds are the active sites of the reaction, and the calculated C-H bond dissociation energy of the CH3CH2C(O)CH(OH)CH2CH3 molecule further confirms that α-CH is the most easily activated. Individual rate constants for five H-abstraction pathways are calculated by canonical variational theory coupled with small curvature tunneling method over the temperature range of 260-330 K, and the branching ratios are also evaluated. A total rate constant of 1.18 × 10-15 cm3 per molecule per s is obtained at 298 K, which is in good agreement with the reported experimental value. A negative temperature dependence is observed in the titular reaction. The subsequent degradation processes of the advantageous product alkyl radical (CH3CH2C˙(OH)COCH2CH3) are carried out in a NO-rich environment, and propionic acid, NO2 and ozone are obtained as the major final products. The nighttime atmospheric lifetime of 4-hydroxy-3-hexanone is estimated to be around 19 days, indicating that it has impact at night. The titular reaction rate constants are fitted to a three-parameter Arrhenius formula.