FAST MUONIUM REACTIONS IN SOLUTION - AN ELECTRON-SPIN EXCHANGE INTERACTION WITH CR(NCS)(6)(3-) IN WATER AND REACTION WITH IODINE IN ORGANIC-SOLVENTS

Two reactions of muonium atoms close to the diffusion-controlled limit were analyzed as a function of solute size and solvent viscosity. Wit h Cr(NCS)(6)(3-) as solute in water, the reaction is an electron spin- conversion process, and the observed rate is taken to be half of the a ctual encounter rate, because of the quantum mechanical statistical fa ctor and the occurrence of multiple collisions due to the solvent cage effect. The encounter rate deduced is 6.2 x 10(10) M-1 s(-1), which i mplies a large cross section for the Cr complex. In a second series of experiments, the rate of reaction of muonium with I-2 was compared in water, methanol and n-heptane. The bimolecular rate constants determi ned are (1.7 +/- 0.3), (7.0 +/- 1.2), and (57 +/- 22) x 10(10) M-1 s(- 1), respectively, in these three solvents. This similar to 30-fold cha nge in rate is not matched by the change in the inverse of the solvent s' viscosities, which changed overall by a factor of only 2.4. It look s as if quantum tunneling dominates over classical diffusion in less p olar media where muonium is unencumbered by solvent clathration.