Steric and repeated-collision effects in diffusion-controlled reactions in solution : kinetics of formation of some iodine donor-acceptor complexes of sulphur compounds

Rates of formation of electron donor–electron acceptor complexes of iodine with some compounds of the general formula R2CS with iodine (R2CS⋯I—I) in 1-chlorobutane have been determined by means of a microwave temperature-jump apparatus. For each of these reactions, the observed forward rate constant (kf) is proportional to the theoretical diffusion-controlled value (k*D) over a range of temperatures, but somewhat smaller. The values of kf/k*D for the various reactions are approximately inversely proportional to the estimated surface areas of the R2CS molecules. Assuming that the area of the reactive site is constant, this suggests that kf/k*D is proportional to the fraction of the molecule surface area that is reactive, i.e. that kf is subject to a geometrical steric factor. The observed values of kf/k*D are, however, ca. four times larger than would be expected on this basis from molecular models, if one took into account only the frequency with which first-time collisions occur between reactant molecules with the correct orientation. This discrepancy can be removed by taking into account the fact that repeated collisions during an encounter will increase the chance of a successful collision. A hard-sphere model of encounters between molecules with limited reaction sites, reorienting by rotation between collisions, suggests that repeated collisions may contribute to kf/k*D a factor in the region of 4, in line with our experimental results.