title: Case of the missing isomer: pathways for molecular elimination in the photoinduced decomposition of 1,1-dibromoethane.
abstract: We report an experimental and computational study of the photodecomposition pathways of a prototypical gem-dihalide, 1,1-dibromoethane (1,1-EDB), in the condensed phase. Following photolysis of the matrix isolated parent compound in Ar at 5 K, photoproducts are observed corresponding to Br2 elimination (+ C2H4 or C2H2) and HBr elimination (+ vinyl bromide). The elimination products are observed in the matrix as complexes. In contrast to our recent studies of the photolysis of matrix isolated polyhalomethanes, no evidence for the iso-1,1-EDB species is found, although studies of the matrix isolated 1,1-dibromo-2,2,2-trifluoroethane analogue show that the isomer is the dominant photoproduct. These results are examined in the light of theoretical studies that have characterized in detail the 1,1-EDB potential energy surface (PES). For Br2 elimination, a pathway from the isomer on the singlet PES is found which involves a simultaneous Br2 loss with 1,2-hydrogen shift; this pathway lies lower in energy than a concerted three-center elimination from the parent 1,1-EDB. For HBr elimination, our previous theoretical studies [Kalume, A.; George, L.; Cunningham, N.; Reid, S. A. Chem. Phys. Lett. 2013, 556, 35-38] have demonstrated the existence of concerted (single-step) and sequential pathways that involve coupled proton and electron transfer, with the sequential pathway involving the isomer as an intermediate. Here, more extensive computational results argue against a simple radical abstraction pathway for this process, and we compare experimental and computational results to prior results from the photolysis of the structural isomer, 1,2-EDB. These steady-state experiments set the stage for ultrafast studies of the dynamics of this system, which will be important in unraveling the complex photodecomposition pathways operative in condensed phases.
authors.affiliation: Department of Chemistry, Marquette University , Milwaukee, Wisconsin 53201-1881, United States.
authors.forename: Aimable
authors.initials: A
authors.lastname: Kalume
authors.forename: Lisa
authors.initials: L
authors.lastname: George
authors.forename: Nicole
authors.initials: N
authors.lastname: Cunningham
authors.forename: Scott A
authors.initials: SA
authors.lastname: Reid
mesh_terms: D015946:Ethylene Dibromide / Q000737:chemistry*; D010782:Photolysis / Q000528:radiation effects*; D011789:Quantum Theory
chemical_list: D015946:Ethylene Dibromide