Reactive Pathways in the Bromobenzene-Ammonia Dimer Cation Radical: Evidence for a Roaming Halogen Radical

Photoinitiated reactions in van der Waals complexes provide a means to examine reactive pathways from well-defined initial geometries. In recent work, we re-examined reactive pathways following resonant two-photon ionization (R2PI) of the chlorobenzene-ammonia (PhCl⋯NH3) dimer. The dimer cation radical reacts primarily via Cl atom loss, and additional channels corresponding to HCl and H atom loss were identified. The structure of the reactive complex was confirmed as an in-plane σ-type, and computational studies of the dimer cation radical potential energy landscape revealed two nearly isoenergetic arenium ion intermediates (or Wheland intermediates). The intermediate produced from ipso addition was not stable with respect to either Cl or HCl loss, and the relative branching observed in experiment was well reproduced by microcanonical transition state theory (TST) calculations. Here, we report experimental and computational studies of the related PhBr⋯NH3 dimer, examined for the first time. We present evidence that the dimer structure is also an in-plane σ-type. However, in contrast to the PhCl⋯NH3 system, calculations predict that the structure of the reactive intermediate corresponds to a distonic ion-radical complex, where the radical density is largely localized on the bromine atom. The calculated barrier to HBr loss is sufficiently high to render this channel nearly insignificant (less than 1% branching) in TST calculations, yet experiment shows a sizable (37%) branching into this channel. We rationalize these results in terms of a roaming Br radical mechanism for HBr formation.