The role of molecular structure in the ˙OH initiated atmospheric oxidation of dibutyl ether.

Dibutyl ether (DBE) and its structural isomers are promising oxygenated fuel components owing to their high cetane numbers and clean combustion characteristics. In the present work, the atmospheric degradation of DBE and its structural isomers, n-dibutyl ether (DiNBE), di-isobutyl ether (DiIBE) and sec-dibutyl ether (DiSBE), initiated by ˙OH is investigated using quantum chemical calculations combined with variational transition state theory. The initial H-abstraction reactions initiated by ˙OH were systematically examined at all distinct abstraction sites for each isomer. The activation strain model indicated that the stabilization of the transition state is primarily governed by the interaction between the reacting fragments at the transition state geometry. For the isomers DiNBE and DiSBE, the H-abstraction at carbon atoms adjacent to the ether oxygen is favourable compared to the other sites. In contrast, for DiIBE, the H-abstraction at the β-carbon atom is found to be favourable. The calculated overall rate constant at 298 K is 1.34 × 10-11, 3.70 × 10-11 and 1.27 × 10-11 cm3 molecule-1 s-1 for DiNBE, DiIBE and DiSBE, respectively. Subsequent reactions of the alkyl radicals formed from favourable H-atom abstraction reactions with O2, NO˙ and HO2˙ lead to the formation of oxygenated products, such as aldehydes, esters, alcohols, hydroperoxides and small carbon-centered radicals. These species play important roles in radical recycling, ozone formation and secondary organic aerosol production. Overall, this work provides detailed mechanistic and kinetic parameters for the ˙OH initiated oxidation of larger dialkyl ethers, enabling more accurate estimation of the atmospheric fate of oxygenated volatile organic compounds through atmospheric chemistry models, and for assessing their environmental impacts.