Non-covalent anion structures in dissociative electron attachment to some brominated biphenyls

The present work combines experiment and theory to reveal the behavior of bromo-substituted-biphenyls after an electron attachment. We experimentally determine anion lifetimes using an electron attachment-magnetic sector mass spectrometer instrument. Branching ratios of dissociative electron attachment fragments on longer timescales are determined using the electron attachment-quadrupole mass spectrometer instrument. In all cases, fragmentation is low: Only the Br^- and [M-Br]^- ions are detected, and [M-H]^- is observed only in the case of 4-Br-biphenyl and parent anion lifetimes as long as 165 µs are observed. Such lifetimes are contradictory to the dissociation rates of 2- and 4-bromobiphenyl, as measured by the pulse radiolysis method to be 3.2 × 10¹⁰ and >5 × 10¹⁰ s^-1, respectively. The discrepancy is plausibly explained by our calculation of the potential energy surface of the dissociating anion. Isolated in vacuum, the bromide anion can orbit the polarized aromatic radical at a long distance. A series of local minima on the potential energy surface allows for a roaming mechanism prolonging the detection time of such weakly bound complex anions. The present results illuminate the behavior recently observed in a series of bromo-substituted compounds of biological as well as technological relevance.