Jm. Stadlbauer et al., FAST MUONIUM REACTIONS IN SOLUTION - AN ELECTRON-SPIN EXCHANGE INTERACTION WITH CR(NCS)(6)(3-) IN WATER AND REACTION WITH IODINE IN ORGANIC-SOLVENTS, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(26), 1997, pp. 4741-4744
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.