Probing the Ignored Elimination Channel of Br₂ in the 248 nm Photodissociation of 1,1‐Dibromoethylene by Cavity Ring‐Down Absorption Spectroscopy

Abstract

In the photodissociation of 1,1-C(2)H(2)Br(2) at 248 nm, the Br(2) elimination channel is probed by using cavity ring-down absorption spectroscopy (CRDS). In terms of spectral simulation, the vibrational population ratio of Br(2)(v = 1)/Br(2)(v = 0) is found to be 0.55+/-0.05, which indicates that the Br(2) fragment is vibrationally hot. The rotational population is thermally equilibrated with a Boltzmann temperature of 349+/-38 K. According to ab initio potential energy calculations, the obtained fragments are anticipated to result primarily from photodissociation of the ground electronic state that undergoes 1) H migration followed by three-center elimination, and 2) isomerization forming either trans- or cis-1,2-C(2)H(2)Br(2) from which Br(2) is eliminated. RRKM calculations predict that the Br(2) dissociation rates through the ground singlet state prevail over those through the triplet state. Measurements of temperature and Ar pressure dependence are examined to support the proposed pathway via internal conversion. The quantum yield for the Br(2) elimination reaction is determined to be 0.07+/-0.04. This result is smaller than that obtained in 1,2-C(2)H(2)Br(2), probably because the dissociation rates are slowed in the isomerization stage.

Photodissociation of Gaseous Acetyl Chloride at 248 nm by Time-Resolved Fourier-Transform Infrared Spectroscopy: The HCl, CO, and CH2 Product Channels

“…The Ar or O 2 addition in the gas-phase photodissociation of CH 3 COCl should facilitate the process of internal conversion with which the HCl, CO, and CH 2 elimination are associated. Analogous phenomena of collision-induced internal conversion have been reported in the cases of acetone , and 1,1-dibromoethylene . The addition of Ar or O 2 facilitates the level-to-level coupling between two electronic states to enhance the radiationless transition.…”

Molecular halogen elimination from halogen-containing compounds in the atmosphere

“…21 In contrast, as with the cases of bromoalkanes, the CRDS spectra of Br 2 (v = 0 and 1) can be readily acquired at 248 nm in the isomeric forms of 1,1-and 1,2-dibromoethylenes. 70,71 The spectrum obtained in 1,1-C 2 H 2 Br 2 is given as an example (Fig. 7).…”

“…8, 1,1-dibromoethylene follows essentially four pathways to the products of HCCH + Br 2 . 71 The route (a) proceeds along i4 (…”

“…The geometries and the harmonic frequencies of the reactant, TS, and products were essentially obtained at the level of the hybrid density functional theory, B3LYP/6-311G(d,p); the energies are further refined by the coupled cluster CCSD(T)/cc-pVTZ with B3LYP/6-311G(d,p) zero-point energy corrections. 71 For some cases, the Br 2 production may not be sufficiently interpreted by the pathways constructed and the subsequent RRKM calculations based on the statistical evaluation. CH 2 BrCOBr as an example gave rise to the Br 2 yield of 0.24 AE 0.08 in photodissociation at 248 nm, 27 about 10 times larger than that by the RRKM calculations while taking into account the other competitive dissociation channels.…”