Photodissociation of dibromoethanes at 248 nm: An ignored channel of Br{sub 2} elimination

Journal Article · Thu May 14 00:00:00 EDT 2009 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.3130768· OSTI ID:21559715

Lee, Hsin-Lung; Lee, Ping-Chen; Tsai, Po-Yu; Lin, King-Chuen ^[1]; Kuo, H H; Chen, P H; Chang, A H. H. ^[2]

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1. Department of Chemistry, National Taiwan University, Taipei 106, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan (China)
2. Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan (China)

Br{sub 2} molecular elimination is probed in the photodissociation of 1,1- and 1,2-C{sub 2}H{sub 4}Br{sub 2} isomeric forms at 248 nm by using cavity ring-down absorption spectroscopy. Their photodissociation processes differ markedly from each other. The quantum yield of the Br{sub 2} fragment in 1,2-dibromoethane is 0.36{+-}0.18, in contrast to a value of 0.05{+-}0.03 in 1,1-dibromoethane. The vibrational population ratios of Br{sub 2}(v=1)/Br{sub 2}(v=0) are 0.8{+-}0.1 and 0.5{+-}0.2 for 1,2- and 1,1-dibromoethanes, respectively. The Br{sub 2} yield densities are found to increase by a factor of 35% and 190% for 1,2- and 1,1-dibromoethanes within the same temperature increment. In the ab initio potential energy calculations, the transition state (TS) along the adiabatic ground state surface may correlate to the Br{sub 2} products. The TS energy for 1,2-dibromoethane is well below the excitation energy at 483 kJ/mol, whereas that for 1,1-dibromoethane is slightly above. Such a small TS energy barrier impedes the photodissociation of the ground state 1,1-dibromoethane such that the production yield of Br{sub 2} may become relatively low, but rise rapidly with the temperature. The TS structure shows a larger bond distance of Br-Br in 1,2-dibromoethane than that in 1,1-dibromoethane. That explains why the former isomer may result in hotter vibrational population of the Br{sub 2} fragments.

• OSTI ID: 21559715
• Journal Information: Journal of Chemical Physics, Vol. 130, Issue 18; Other Information: DOI: 10.1063/1.3130768; (c) 2009 American Institute of Physics; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

74 ATOMIC AND MOLECULAR PHYSICS
37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ABSORPTION SPECTROSCOPY
BOND LENGTHS
BROMINE
DENSITY
DISSOCIATION
EXCITATION
GROUND STATES
ISOMERS
ORGANIC COMPOUNDS
PHOTOCHEMISTRY
PHOTOLYSIS
PHOTON-MOLECULE COLLISIONS
POTENTIAL ENERGY
SURFACES
ULTRAVIOLET SPECTRA
CHEMICAL REACTIONS
CHEMISTRY
COLLISIONS
DECOMPOSITION
DIMENSIONS
ELEMENTS
ENERGY
ENERGY LEVELS
ENERGY-LEVEL TRANSITIONS
HALOGENS
LENGTH
MOLECULE COLLISIONS
NONMETALS
PHOTOCHEMICAL REACTIONS
PHOTON COLLISIONS
PHYSICAL PROPERTIES
SPECTRA
SPECTROSCOPY