Preferential C–Cl bond rupture from 1-bromo-2-chloro-1,1,2-trifluoroethane following photoabsorption via n(Cl)→σ*(C–Cl) transition

Abstract

The photodissociation mechanism and dynamics of the title molecule have been studied at excitation wavelengths of 157 and 193 nm by using a photofragmentation translational spectroscopy. In the case of the excitation at 157 nm, the C–Cl and C–Br bond ruptures occur competitively with the branching ratio of 1.0:0.3, indicating the preferential C–Cl bond rupture over the weaker C–Br bond. The C–Br bond rupture occurred via two pathways; the dissociation on the excited repulsive potential energy surface and the dissociation of the vibrationally excited molecule in the ground electronic state following the internal conversion. In the case of 193 nm, only the C–Br bond rupture on the repulsive potential energy surface was observed.

UV Photodissociation of Halothane in a Focused Molecular Beam: Space-Speed Slice Imaging of Competitive Bond Breaking into Spin–Orbit-Selected Chlorine and Bromine Atoms

“…The branching ratio of chlorine versus bromine yields is about two-to-one. The further acquired results and interpretation given here, with respect to those in earlier refs and , were made possible by our effective use of the hexapolar field, which works both through focusing and enhancing the molecular beam intensity and also as a filter against cluster contamination and excited conformer contributions. , The relative yields for branching of chlorine and bromine require to be considered by taking into account not only of the correlated absorption states but also of the role of nonadiabatic coupling between the two involved interacting surfaces, n σ*­(C–Cl) and n σ*­(C–Br), arguably arising from the cuts of conical intersections. This benchmark work can be extended to shorter wavelengths as a step toward the study of three-body photodissociation, opening the threshold, for example, to further dissociation products: the yield of chlorine atoms, having been found ca.…”

“…The chlorine atoms are the dominant product: the main contribution of Cl atoms is from the coupled states interacting nonadiabatically with the channel leading to direct Br formation. Our results provide modern experimental support to a pioneer report: Yokoyama et al 17,28 have studied the photodissociation of halothane at 157 nm and found a branching ratio of 1:0.75 for Cl:Br dissociation, the main products being chlorine atoms. They endorsed the interpretation of the relative yield of Cl and Br formation as stemming from the nonadiabatic interaction between the nσ*(C−Cl) and the nσ*(C−Br) surface.…”

“…Tzeng et al 16 reported that for a 193 nm photodissociation of CH 2 BrCl, the branching ratio of C−Cl versus C−Br bond breakings was 4.5: C−Cl bond fission was found to be favored over that of the weaker C−Br bond. Yokoyama et al 17 Cl atoms about 3 times that of Br atoms. A similar unexpected tendency has also been reported for bromoacetyl chloride 18,19 and bromopropionyl chloride.…”

“…reported that for a 193 nm photodissociation of CH 2 BrCl, the branching ratio of C–Cl versus C–Br bond breakings was 4.5: C–Cl bond fission was found to be favored over that of the weaker C–Br bond. Yokoyama et al demonstrated preferential fission of the stronger C–Cl bond over that of the C–Br bond in the photodissociation of CBrF 2 CHClF at 193 nm, with a yield of Cl atoms about 3 times that of Br atoms. A similar unexpected tendency has also been reported for bromoacetyl chloride , and bromopropionyl chloride .…”

A vector correlation study using a hexapole-oriented molecular beam: photodissociation dynamics of oriented isohaloethane

“…Yokoyama and co-workers previously examined the photodissociation of isohaloethane at 157 and 193 nm. 24 They found that the branching ratio of Cl and Br formation upon excitation at 157 nm was 1.0 : 0.3, indicating preferential C–Cl bond cleavage despite the weaker C–Br bond. In contrast, only Br atom formation was observed at 193 nm.…”