Facile four-centre exchange reactions

Abstract

Molecular beam experiments show that diatomic molecule exchange reactions involving an ionic bond proceed readily at thermal collision energies. Previous work found two such reaction families, alkali halide + alkali halide and dialkali + halogen. This study adds a third, alkali halide + halogen. For CsI + Cl₂→ CsCl + ICl the product angular and velocity distributions indicate that a statistical collision complex is formed which persists for many vibrational periods and at least a few rotational periods. This complex presumably corresponds to the alkali trihalide salt, Cs⁺(ClICl)^–. For the CsBr + ICl reaction only formation of CsCl + IBr has been observed, even at collision energies well above the endoergic threshold for formation of CsI + BrCl. The energy disposal is not statistical and the product angular distribution is quite asymmetric about 90°, indicating that a large fraction ([graphic omitted] ⅓ to ½) of the collision complexes break up in less than one rotational period. The preferred directions of emission are 0° for CsCl and 180° for IBr, where 0° and 180° designate the incident beam directions (c.m. system) for ICl and CsBr, respectively. These properties can be plausibly interpreted in terms of the electronic structure of the trihalide group.