Photodissociation of dibromoethanes at 248 nm: An ignored channel of Br2 elimination

Abstract

Br(2) molecular elimination is probed in the photodissociation of 1,1- and 1,2-C(2)H(4)Br(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(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(2)(v=1)/Br(2)(v=0) are 0.8+/-0.1 and 0.5+/-0.2 for 1,2- and 1,1-dibromoethanes, respectively. The Br(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(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(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(2) fragments.

When isomerisation is electron transfer: the intriguing story of the iso-halocarbons

“…The gas-phase photolysis of 1,2-dibromoethane (hereafter referred to 1,2-EDB) has been studied extensively in the gas phase [142][143][144], and like the halomethanes, not without some controversy concerning the decomposition pathways. In this case, the bromoethyl radical adopts a classical, carbon-centred structure and molecular beam experiments using photofragment translational spectroscopy with photolysis at wavelengths of 234 nm [145,146], 248 nm [144] and 267 nm [145,146] are consistent in showing a three body asynchronous dissociation producing ethylene and two Br atoms.…”

“…In this case, the bromoethyl radical adopts a classical, carbon-centred structure and molecular beam experiments using photofragment translational spectroscopy with photolysis at wavelengths of 234 nm [145,146], 248 nm [144] and 267 nm [145,146] are consistent in showing a three body asynchronous dissociation producing ethylene and two Br atoms. However, in bulk gas samples at low pressure, Lin and co-workers have identified a Br 2 elimination pathway with a quantum yield of~0.4, which was suggested to occur via a four-centre TS on the ground state PES [143]. We recently studied the photolysis of 1,2-EDB in inert gas matrices at low temperature [86], and found that Br 2 and C 2 H 4 were the primary photoproducts following irradiation with 220 nm laser light [147].…”

“…Molecular beam studies of 1,1-dibromoethane show that 248 nm excitation leads to C-Br bond fission [144]; however, in bulk gas samples at low pressure, Lin and co-workers identified a Br 2 elimination pathway with a quantum yield of 0.05, which was suggested to occur via a three-centre TS on the ground state PES [143]. Molecular beam studies of 1,1-dibromoethane show that 248 nm excitation leads to C-Br bond fission [144]; however, in bulk gas samples at low pressure, Lin and co-workers identified a Br 2 elimination pathway with a quantum yield of 0.05, which was suggested to occur via a three-centre TS on the ground state PES [143].…”

“…Recall that Lin and co-workers identified a Br 2 elimination pathway in gas-phase photolysis experiments on the dibromoethanes, with a reported quantum yield of~0.4 for 1,2-EDB, and a much smaller quantum yield of~0.05 for 1,1-EDB [143]. Recall that Lin and co-workers identified a Br 2 elimination pathway in gas-phase photolysis experiments on the dibromoethanes, with a reported quantum yield of~0.4 for 1,2-EDB, and a much smaller quantum yield of~0.05 for 1,1-EDB [143].…”

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

“…The photodissociation of polyhalogenalkanes induced by intense laser elds has been extensively studied and is the subject of recent reviews. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Two major reactions are possible for dissociation of polyhalogenalkanes: the halogen atom release reaction channel and the halogen molecular elimination reaction channel. Thus far, most studies have been focused on the halogen atomic product channel, and the halogen molecular elimination channel as a research topic has received less attention in earlier papers.…”