Photodissociation of CH2BrCHBrC(O)Cl at 248 nm: probing Br2 as the primary fragment using cavity ring-down spectroscopy

Abstract

The photodissociation of 2,3-dibromopropionyl chloride (CH₂BrCHBrC(O)Cl, 2,3-DBPC) at 248 nm was carried out to study Br₂ as the primary molecular product in the B³Π⁺_0u ← X¹Σ⁺_g transition using cavity ring-down absorption spectroscopy. The rotational spectra (v′′ = 0–2) were acquired and assigned with the aid of spectral simulation. It is verified that the obtained Br₂ fragment is attributed to the one-photon dissociation of 2,3-DBPC and is free from contributions of secondary reactions. The vibrational ratio of the Br₂ population of v(0):v(1):v(2) is equal to 1:(0.58 ± 0.12):(0.23 ± 0.09), corresponding to the Boltzmann vibrational temperature of 623 ± 38 K. The quantum yield of Br₂ eliminated from 2,3-DBPC is estimated to be 0.09 ± 0.04. The dissociation pathways of 2,3-DBPC and its potential energy surfaces were calculated using density functional theory. By employing the CCSD(T)//M062X/6-31+g(d,p) level of theory, transition state barriers and corresponding reaction energies were calculated for the Br, Cl, Br₂, BrCl, HBr and HCl elimination channels. The unimolecular rate constant for Br₂ elimination was determined to be 2.09 × 10⁵ s⁻¹ using Rice–Ramsperger–Kassel–Marcus (RRKM) theory, thus explaining the small quantum yield of the Br₂ channel.