Formation and decay of an aromatic vinyl cation in supercritical CF₃H

The effect of local density enhancement on the diffusion-controlled reactions in supercritical (SC) fluids has been clearly detected in the recombination reaction of a vinyl cation, 1-(4-methoxyphenyl)-2,2-diphenyl cation, with Br^- in SC CF₃H. With increasing density of SC CF₃H, the observed recombination rate as a whole decreased rapidly in proportion to 1/ηε, where η and ε are the bulk viscosity and dielectric constant of the medium, respectively. Near the critical density, the rate constant was found to be much smaller than the predicted value from the 1/ηε relation based on the Smolchowski-Debye scheme. This deviation indicates that the local density enhancement retards the recombination rate mainly due to the enhancement of dielectric constant.