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

The photodissociation of 2,3-dibromopropionyl chloride (CH2BrCHBrC(O)Cl, 2,3-DBPC) at 248 nm was carried out to study Br-2 as the primary molecular product in the B-3 pi(+)(0u) <- X-1 sigma(+)(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 Br-2 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 Br-2 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 Br-2 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, Br-2, BrCl, HBr and HCl elimination channels. The unimolecular rate constant for Br-2 elimination was determined to be 2.09 x 10(5) s(-1) using Rice-Ramsperger-Kassel-Marcus (RRKM) theory, thus explaining the small quantum yield of the Br-2 channel.