Photodissociation of dibromoethanes at 248 nm: An ignored channel of Br[sub 2] elimination

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 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.

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

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.

BrCl+ elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields

By using a dc-slice imaging technique, photodissociation of 1,2-C₂H₄BrCl was investigated at 800 nm looking for heteronuclear unimolecular ion elimination of BrCl⁺ in an 80 fs laser field. The occurrence of fragment ion BrCl⁺ in the mass spectrum verified the existence of a unimolecular decomposition channel of BrCl⁺ in this experiment. The relative quantum yield of the BrCl⁺ channel was measured to be 0.8%. By processing and analyzing the velocity and angular distributions obtained from the corresponding sliced images of BrCl⁺ and its partner ion C₂H₄⁺, we concluded that BrCl⁺ came from Coulomb explosion of the 1,2-bromochloroethane dication 1,2-C₂H₄BrCl²⁺. With the aid of quantum chemical calculations at the M06-2X/def2-TZVP level, the potential energy surface for BrCl⁺ detachment from 1,2-C₂H₄BrCl²⁺ has been examined in detail. According to the ab initio calculations, two transition state structures tended to correlate with the reactant 1,2-C₂H₄BrCl²⁺ and the products BrCl⁺ + C₂H₄⁺. In this entire dissociation process, the C–Br and C–Cl bond lengths were observed to elongate asymmetrically, that is, the C–Br chemical bond broke firstly, and subsequently a new Br–Cl chemical bond started to emerge while the C–Cl bond continued to exist for a while. Hence, an asynchronous concerted elimination mechanism was favored for BrCl⁺ detachment.

Concerted elimination of the molecular ion BrCl⁺ from Coulomb explosion of 1,2-bromochloroethane was studied theoretically and experimentally.

Characterization of molecular channel in photodissociation of SOCl2 at 248 nm: Cl2 probing by cavity ring-down absorption spectroscopy

A primary elimination channel of the chlorine molecule in the one-photon dissociation of SOCl2 at 248 nm was investigated using cavity ring-down absorption spectroscopy (CRDS). By means of spectral simulation, the ratio of the vibrational population in the v = 0, 1, and 2 levels was evaluated to be 1 : (0.10 ± 0.02) : (0.009 ± 0.005), corresponding to a Boltzmann vibrational temperature of 340 ± 30 K. The Cl2 molecular channel was obtained with a quantum yield of 0.4 ± 0.2 from the X(1)A' ground state of SOCl2via internal conversion. The dissociation mechanism differs from a prior study where a smaller yield of <3% was obtained, initiated from the 2(1)A' excited state. Temperature-dependence measurements of the Cl2 fragment turn out to support our mechanism. With the aid of ab initio potential energy calculations, two dissociation routes to the molecular products were found, including one synchronous dissociation pathway via a three-center transition state (TS) and the other sequential dissociation pathway via a roaming-mediated isomerization TS. The latter mechanism with a lower energy barrier dominates the dissociation reaction.

Molecular halogen elimination from halogen-containing compounds in the atmosphere

Atmospheric halogen chemistry has drawn much attention, because the halogen atom (X) playing a catalytic role may cause severe stratospheric ozone depletion. Atomic X elimination from X-containing hydrocarbons is recognized as the major primary dissociation process upon UV-light irradiation, whereas direct elimination of the X2 product has been seldom discussed or remained a controversial issue. This account is intended to review the detection of X2 primary products using cavity ring-down absorption spectroscopy in the photolysis at 248 nm of a variety of X-containing compounds, focusing on bromomethanes (CH2Br2, CF2Br2, CHBr2Cl, and CHBr3), dibromoethanes (1,1-C2H4Br2 and 1,2-C2H4Br2) and dibromoethylenes (1,1-C2H2Br2 and 1,2-C2H2Br2), diiodomethane (CH2I2), thionyl chloride (SOCl2), and sulfuryl chloride (SO2Cl2), along with a brief discussion on acyl bromides (BrCOCOBr and CH2BrCOBr). The optical spectra, quantum yields, and vibrational population distributions of the X2 fragments have been characterized, especially for Br2 and I2. With the aid of ab initio calculations of potential energies and rate constants, the detailed photodissociation mechanisms may be comprehended. Such studies are fundamentally important to gain insight into the dissociation dynamics and may also practically help to assess the halogen-related environmental variation.

Coulomb explosion and dissociative ionization of 1,2-dibromoethane under an intense femtosecond laser field

Coulomb explosion and dissociative ionization of 1,2-dibromoethane are experimentally investigated in a near-infrared (800 nm) femtosecond laser field by dc-slice imaging technology.