Role of (H ₂ O) _n (n = 1-2) in the Gas-Phase Reaction of Ethanol with Hydroxyl Radical: Mechanism, Kinetics, and Products

The effect of water on the hydrogen abstraction mechanism and product branching ratio of CH ₃ CH ₂ OH + ^a OH reaction has been investigated at the CCSD(T)/aug-cc-pVTZ//BH&HLYP/aug-cc-pVTZ level of theory, coupled with the reaction kinetics calculations, implying the harmonic transition-state theory. Depending on the hydrogen sites in CH ₃ CH ₂ OH, the bared reaction proceeds through three elementary paths, producing CH ₂ CH ₂ OH, CH ₃ CH ₂ O, and CH ₃ CHOH and releasing a water molecule. Thermodynamic and kinetic results indicate that the formation of CH ₃ CHOH is favored over the temperature range of 216.7-425.0 K. With the inclusion of water, the reaction becomes quite complex, yielding five paths initiated by three channels. The products do not change compared with the bared reaction, but the preference for forming CH ₃ CHOH drops by up to 2%. In the absence of water, the room temperature rate coefficients for the formation of CH ₂ CH ₂ OH, CH ₃ CH ₂ O, and CH ₃ CHOH are computed to be 5.2 × 10 ^-13 , 8.6 × 10 ^-14 , and 9.0 × 10 ^-11 cm ³ molecule ^-1 s ^-1 , respectively. The effective rate coefficients of corresponding monohydrated and dihydrated reactions are 3-5 and 6-8 orders of magnitude lower than those of the unhydrated reaction, indicating that water has a decelerating effect on the studied reaction. Overall, the characterized effects of water on the thermodynamics, kinetics, and products of the CH ₃ CH ₂ OH + ^a OH reaction will facilitate the understanding of the fate of ethanol and secondary pollutants derived from it.