Molecular Beam Kinetics: Four-Atom Collision Complexes in Exchange Reactions of CsCl with KCl and KI

Abstract

Angular distributions of scattering from crossed molecular beams have been measured for CsCl+KCl and CsCl+KI and velocity distributions for the latter case. The parent beam temperatures are ∼ 1000°K, corresponding to mean collision energy of ∼ 4 kcal/mole and mean total energy of ∼ 11 kcal/mole. Both systems show ``sticky collision bumps'' at wide angles, indicating formation of collision complexes with lifetime at least comparable to a rotational period ( ≳ 10−12 sec). The cross sections for complex formation are roughly ≳ 200 Å2. The surface ionization detector used does not distinguish between products and reactants for these systems (all salts yield atomic alkali ions). However, the kinematic effect of the mass change in the CsCl + KI → CsI + KCl reaction allows the reactive and nonreactive contributions to the velocity distributions to be resolved. Reactive decay is estimated to occur in about one out of every three or four collisions in which a complex is formed. The velocity analysis also shows that the relative translational energy of the products is distinctly less than (∼ 60%) that of the reactants, and less than that for decay of comparable three-atom complexes. The data are compared with a simple statistical, transition-state theory for a four-atom complex. As a consequence of the unusually strong long-range dipole-dipole interaction, this essentially requires no molecular parameters other than the number of degrees of freedom, masses, bond strengths, and dipole moments. The statistical theory gives good agreement with the observed translational energy and angular distributions, but apparently overestimates the ratio of reactive to nonreactive decay of the complex. This may be due to geometrical isomerism. Ionic model calculations predict two stable isomeric configurations of alkali halide dimers. The more stable isomer is a cyclic, planar rhomboid; it provides a pathway for the concerted making and breaking of pairs of bonds in the exchange reaction. The less stable isomer is a linear chain; it may often dissociate nonreactively rather than rearrange to the cyclic form, especially in collisions with large impact parameters, when the centrifugal momentum is very large and thus keeps the chain ``ends'' apart.