Hydrated Electron Reactions with Thiols in Acidic Aqueous Solutions

The rates of hydrated electron reactions with cysteamine, cysteine, thioglycol, α-mercaptopropionic acid, β-mercaptopropionic acid, and penicillamine have been studied in the pH 1-3 region. Hydrogen yields, which are a function of pH in this region, were used to monitor the competition between H+ and RSH for hydrated electrons. Corrections were applied to the observed rate constants to account for dissociation of the COOH group and the ionic strength effect on hydrated electron reactions. In the cases of cysteine and penicillamine, the rate constants for reaction with protonated amino acids at infinite dilution were greater by a factor of 3-5 than those for reaction with the zwitterion. Uncharged species show no pH dependence of the rate constant within this region. All rate constants measured were of comparable magnitude and approached the diffusion controlled limit.