Symmetry Control in Bond Cleavage Processes: Dissociative Electron Attachment to Unsaturated Halocarbons

Abstract

Dissociative electron attachment (DEA or DA) to chlorobenzene, benzyl chloride, o-, m- and p-tolyl chloride, vinyl chloride, and allyl chloride has been investigated, including an analysis of the kinetic energy content of the fragments, i.e. hydrocarbon radical and Cl^⊖. Electron transmission (ET) spectroscopy is simultaneously employed to reveal the properties of the initial temporary molecular anions. The results suggest that for all systems the incident electron is captured into a low-lying π*-orbital. If the C–Cl bond lies in the nodal plane of the π-system the symmetry of the ground state fragments does not correlate with the initial anion resonance of ²Π-symmetry. Predissociation offers a mechanism to yield these products but sufficient time remains to channel the excess energy into various internal degrees of freedom of the products, resulting experimentally in less kinetic energy release. Alternatively, if the C–Cl bond does not lie in the nodal plane of the π-system, adiabatic correlation between initial and final species in their ground states is obtained. The initial anion is formed in a repulsive state and Cl^⊖ appears experimentally with considerable kinetic energy. The present results thus offer clear-cut experimental indications for symmetry control in bond cleavage processes.