Investigating the Photooxidation of CHBr₃: Matrix-Isolation, Gas-Phase, and DFT Study

Bromoform (CHBr₃) is a very short-lived substance (VSLS) present in the troposphere, originating mainly from natural sources (e.g., phytoplankton and macroalgae)¹. It contributes to the formation of reactive bromine species that participate in ozone depleting and other atmospheric reactions². The aim of this work is to investigate the evolution of CHBr₃ in the presence of O₂ under UV-vis irradiation and to elucidate the mechanism involved in the photoreaction.

Different CHBr₃:O₂:Ar mixtures (1:1:400 and 1:20:400) were prepared on a vacuum line using standard manometric techniques. The mixtures were deposited on a CsI window cooled at 10 K using a pulse-deposition technique. A Xe(Hg) arc lamp was operated at 800 W. To reproduce atmospheric conditions, selected spectral intervals were used: 400≤λ≤ 800 nm for visible radiation; 350≤λ≤450 nm and 280≤λ≤320 nm, corresponding to portions of the UV-A and UV-B regions, respectively; and 200≤λ≤800 nm to include UV-C, UV-B, UV-A, and visible radiation. Complementarily, we investigated the reaction mechanism between CHBr₃ and O₂ in the gas phase using a home-built cell that allows simultaneous irradiation and FTIR acquisition.

The initial spectra of the matrix-isolated samples showed only the bands of CHBr₃. After irradiation with broadband UV-vis light and using the 350≤ λ ≤ 450 nm filter, new bands appeared in the absorption region of CO, HBr, and CO₂. These bands exhibited the same growth kinetics and increased their intensity after annealing. CO, HBr, and CO₂ were detected as photoproducts in gas phase experiments.

The structures of the possible intermolecular complexes formed among the photoproducts were theoretically simulated using the B3LYP-D3/6-311++G(d,p) approximation. At the molecular level, we detected, via FTIR-matrix isolation techniques, new complexes involving CO, HBr, and Br₂, in agreement with theoretical calculations.

(1) Villamayor et al. Nat. Clim. Chang. 13 (2023) 554–560.

(2) Tinel et al. Elem. Sci. Anthr. 11:1 (2023) 00032.