Applications of the channel flow cell for UV-visible spectroelectrochemical studies Part 3. Do radical cations and anions have similar diffusion coefficients to their neutral parent molecules?

Abstract

Abstract A chronoabsorptometric method has been used to determine the diffusion coefficients of electrogenerated radical ions from a wide range of compounds. The technique was first verified using the well-established ferro/ferricyanide system and excellent agreement between experiment and literature noted. The effect of solvent on the diffusion coefficient was exemplified by means of the N,N,N′,N′-tetramethyl-p-phenylenediamine radical cation in water, ethanol and acetonitrile. It was found that the diffusion coefficients of the radical cation in water (8.00 × 10−6 cm2 s−1) and ethanol (8.23 × 10−6 cm2 s−1) are 5 to 15% smaller than those of the parent molecules, while in acetonitrile the diffusion coefficient (2.02 × 10−5 cm2 s−1) is almost equal to that of the parent species. In addition, diffusion coefficients for the 2,3,5,6-tetrabromo-1,4-benzoquinone radical anion (1.86 × 10−5 cm2 s−1), 2,3,5,6-tetrachloro-1, 4-benzoquinone radical anion (2.00 × 10−5 cm2 s−1) and phenothiazine radical cation (1.97 × 10−5 cm2 s−1) have been measured in acetonitrile at 25°C and are shown to be virtually identical to those of the parent compounds. It is suggested that, for species such as these, the interaction between the radical ion species and the solvent molecules resembles that of the uncharged parent molecule resulting in a diffusion rate very close to that of the latter.