Muonium has been used as an H atom analogue to investigate the free radicals formed by H addition to the polyaromatic hydrocarbon fluoranthene. There are nine unique carbons in the molecule, but only five radicals were detected. Muon and proton hyperfine constants were determined by transverse field µSR and µLCR, respectively, and compared with calculated values. All signals were assigned to radicals formed by Mu addition to C-H sites. There isno evidence for addition to the tertiary carbons at ring junctions.
Rate constants are reported for near-diffusion-controlled reactions of muonium in sub- and supercritical water. Specifically, the spin-exchange interaction of muonium with Ni2 + and the addition of muonium to hydroquinone were studied as a function of temperature and pressure over a wide range of conditions, from standard to over 400 °C and 400 bar (the critical point of water is at 374 °C, 220 bar). At elevated temperatures the rate constants were found to have values far below those predicted by Stokes–Einstein–Smoluchowski theory. Furthermore, the temperature variation of the isobaric rate constants has a maximum in the subcritical region. The pressure dependence of the rate constants increases with temperature, consistent with the increase in compressibility of the solvent; the effective activation volumes are negative. Various models are explored to interpret the temperature and density dependence of the kinetic data. It is concluded that a key factor in the drop of rate constants at high temperature is the cage effect, in particular the number of collisions between a pair of reactants over the duration of their encounter.
