Kinetics of Mu addition to acetone in sub- and supercritical water

The rate constant for the addition of muonium to acetone in aqueous medium has been measured in sub- and supercritical conditions at 250 bar in the temperature range 100–400 °C using the transverse field muon spin rotation (TF-μSR) technique. At temperatures up to ≈200 °C, the reaction shows the expected Arrhenius behaviour. However above ≈250 °C, the rate constant goes through a maximum then drops rapidly between 300 and 400 °C. This adds to a list of similar observations made for muonium reacting with very different substrates, indicating that the decrease of the value of the rate constant with increasing temperature above supercritical conditions may be a general phenomenon in aqueous solution. Allowing muonium to react with both the keto and enol form of acetone, each with a different rate constant ( k O and k E, respectively), and using various constraints on the kinetic parameters of the system, an empirical fit to the present data provides an estimate of K E, the enolization equilibrium constant of acetone. At 350 °C, one finds k O≈9×10 7 M −1 s −1, 1.0×10 10< k E