Ab Initio and RRKM/Master Equation Analysis of the Photolysis and Thermal Unimolecular Decomposition of Bromoacetaldehyde

Bromoacetaldehyde (BrCH₂CHO) is a major stable brominated organic intermediate of the bromine–ethylene addition reaction during the arctic bromine explosion events. Similar to acetaldehyde, which has been recently identified as a source of organic acids in the troposphere, it may be subjected to photo-tautomerization initially forming brominated vinyl compounds. In this study, we investigate the unimolecular reactions of BrCH₂CHO under both photolytic and thermal conditions using high-level quantum chemical calculations and Rice–Ramsperger–Kassel–Marcus (RRKM)/master equation analysis. The unimolecular decomposition of BrCH₂CHO takes place through 14 dissociation and isomerization channels along a potential energy surface involving eight wells. Under the assumption of singlet ground-state potential energy surface-dominated photodynamics, the primary photodissociation yields of BrCH₂CHO are investigated under both collision-free and collision energy transfer conditions. At atmospheric pressure and under tropospheric actinic flux conditions at ground level, depending on the assumed collisional energy transfer parameter, 150 cm^–1 < ⟨ΔE_down⟩ < 450 cm^–1, 78–33% of BrCH₂CHO undergoes direct photodissociation instead of collisional deactivation at an excitation wavelength of 320 nm. This is significantly higher than the 14% reported for acetaldehyde, hence indicating a strong effect of bromine substitution on the product photolysis yield that is related to additional favorable Br and HBr forming dissociation channels. In contrast to the overall photodissociation quantum yield, the relative branching fractions of the photodissociation products are less dependent on the collisional energy transfer parameter. For a representative value of ⟨ΔE_down⟩ = 300 cm^–1 and an excitation wavelength of 320 nm, with 27% for C–C bond fission, 11% for C–Br bond fission, 7% for HBr elimination, and only below 2% each for a consecutive O–Br fission reaction and the photo-tautomerization channel yielding brominated vinyl alcohol, the photodissociation is markedly different from the acetaldehyde case. Finally, as brominated halogenated compounds are of interest for flame inhibition purposes, thermal multichannel unimolecular rate constants were calculated for temperatures in the range from 500 to 2000 K. At a temperature of 2000 K and ambient pressure, the two main reaction channels are the C–Br and C–C bond fissions, contributing 35 and 43% to the total reaction flux, respectively.