Viscosity Dependence of Iodine Recombination

Abstract

Iodine photodissociation provided one of the original examples of the cage effect in liquids. Since then, the concept of a cage has become of central importance to chemists in guiding and understanding their choice of solvents for particular chemical reactions. Recently, HOFFMAN, et al. [1] applied picosecond techniques to study the dissociation and subsequent recombination of I in CCl4 and hexadecane. They showed that the recombination occurs in two time scales, a very short (around 100 nsec) and a much longer one (greater than l nsec.). This was interpreted as a direct demonstration of the cage effect. The short time decay is the recombination of caged radicals (geminate) and the long time decay is due to recombination of randomly diffusing I atoms (nongeminate). This experiment provides a good opportunity to test various diffusion theories, brownian motion theories, and molecular dynamics descriptions of translational motion in liquids. Thus, we have extended these experiments to study the recombination as a function of solvent viscosity and structure.