Probing BrCl from photodissociation of CH2BrCl and CHBr2Cl at 248 nm using cavity ring-down spectroscopy

Abstract

Photodissociation of di- and tri-halogenated methanes including CH₂BrCl and CHBr₂Cl at 248 nm was investigated using cavity ringdown absorption spectroscopy (CRDS). The spectra of the BrCl(v′′ = 2, 3) and Br₂(v′′ = 1, 2) fragments were probed over the wavelength range of 594.5–596 nm in the B³Π⁺_0u ← X¹Σ⁺_g and B³Π (0⁺) ← X¹Σ⁺ transitions, respectively. Their corresponding spectra were simulated for assignment of rotational lines at a given vibrational level. The quantum yields for Br₂ eliminated from CHBr₂Cl and BrCl from CH₂BrCl were determined to be 0.048 ± 0.018 and 0.037 ± 0.014, respectively. The photodissociation of CHBr₂Cl yielded only the Br₂ fragment, but not the BrCl fragment in the experiments. An ab initio theoretical method based on the CCSD(T)//B3LYP/6-311g(d,p) level was employed to evaluate the potential energy surface for the dissociation pathways to produce Br₂ and BrCl from CHBr₂Cl, which encountered a transition state barrier of 445 and 484 kJ mol⁻¹, respectively. The corresponding RRKM rate constants were calculated to show that the branching ratio of (Br₂/BrCl) is ∼20. The BrCl spectrum is expected to be obscured by the much larger Br₂ spectrum, explaining why BrCl fragments cannot be detected in the photolysis of CHBr₂Cl.