The Kinetics of the Recombination of Methyl Radicals and Iodine Atoms

Abstract

The process of recombination of free radicals may be formally regarded as proceeding via an intermediate complex (the so-called activated complex) in which the radicals are more or less rigidly bound together to form an "active" molecule. The term "active" here denotes a molecule containing a large excess of vibrational energy arising from the formation of the new bond. This excess energy must be removed through a deactivating collision, else it will reaccumulate in the new bond, and the molecule will decompose shortly after it has been formed. This study treats the two main problems presented by radical recombinations: the magnitude of the steric effects tending to reduce the rate below that calculated by the kinetic theory of collisions (this effect gives rise to the so-called "steric" factor) and the effect of pressure on the rate of recombination. For example, if the activated complex is a rigid structure, i.e., the radicals are rigidly bound together, some of the rotational degrees of freedom of the radicals in their free state must be "frozen out" into bending vibrations of the new bond so that the activated complex may be formed. The relatively small probability of such a process results in a reduced chance of the formation of the activated complex and thus not every collision of radical and atom is effective in producing an active molecule. If, on the other hand, the activated complex has a loose structure, i.e., one in which the radicals rotate freely, then there will be no such restrictions on the rate of formation of the activated complex and every collision will be effective in producing an active molecule.

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