Crossed-beam slice imaging of Cl reaction dynamics with butene isomers.

We present a crossed-beam imaging study of the reaction of chlorine atoms with several butene isomers. A high-intensity pulsed ablation Cl source is used with DC slice imaging and single-photon ionization detection at 157 nm to record the velocity-flux contour maps for these reactions. The target unsaturated hydrocarbons are 1-butene, trans-2-butene, cis-2-butene, and isobutene (2-methylpropene). Data are obtained at collision energies of ~13.0 kcal·mol(-1). Distinct differences in the scattering distributions and in particular the coupling of angular and translational energy release provide insight into the dynamics of this little-studied class of reactions. We find that these distributions reflect the energetics for competition between addition/elimination and direct abstraction in line with ab initio thermochemical data. A possible role for Cl atom roaming mediating the addition/elimination pathway is suggested.