Kinetics and mechanisms of some combination and decomposition reactions.

Abstract

Atom and free radical combinations in the gas phase are considered on the basis of (i) the energy transfer mechanism, in which the two atoms or radicals form a complex which is deactivated by a collision with a foreign molecule; (ii) the radical molecule complex mechanism, in which the atom or radical forms a complex with the foreign molecule; this complex is deactivated by collisions and finally undergoes a reaction with a second atom or free radical. Estimates are made of the binding energies between iodine atoms and various chaperon molecules, dispersion forces and charge transfer being taken into account. Calculations are made of the rates of iodine atom combinations in the presence of a variety of chaperons. For certain simple molecules, including the inert gases, there is little binding due to either charge transfer or to the quantum-mechanical dispersion forces. The binding energies for these molecules are less than the average thermal energy; the complexes therefore cannot be deactivated, and can play no role in atom combinations. For these systems, the energy transfer mechanism gives good agreement between theory and experiment. With n-butane, benzene and molecular iodine as chaperons, on the other hand, the rates are higher than can be explained on the basis of energy transfer; complex formation, involving dispersion forces and charge transfer, therefore plays a role in the reactions.