Kinetic Study of the Fast Halogen-Trihalide Ion Equilibria in Protic Media by the Raman-Laser Temperature-Jump Technique. A Non-Diffusion-Controlled Ion-Molecule Reaction

Article abstract of DOI:10.1021/j100409a034

Forward (k+) and reverse (k-) rate constants of the X2 + X(1-) <*> X3(1-) equilibrium in water, methanol, ethanol, and acetic acid for X=Br and in water and methanol for X=I have been measured by the Raman-laser temperature-jump technique.For X=Br chemical relaxation signals are always very fast, in the 20-75-ns range; the relaxation times are then obtained by deconvoluting the experimentally observed signals by the response function of the apparatus.Rate constants k+ are found to be around 1E9 M^-1s^-1, and k- values vary from 6E5 to 5E7 s^-1, depending on the solvent and the halogen.When X=I the k+ values approach the diffusion-controlled limit, whereas for X=Br these values are significantly smaller than the diffusion rates evaluated by the Smoluchowski equation.The slope of the rate-equilibrium relationship, which is near zero when X=I but 0.25 for X=Br, supports this conclusion.The addition of an iodide ion to iodine is almost diffusion-controlled but that of bromide is clearly activation-controlled.This result is discussed in terms of the dynamics of halide ion solvatation, which could be an important rate-determining factor, as supported by the linear relationship between the k+ rates and the free energies of halide ion transfer.