E. Lazzarini et al., ELECTRON-SPIN EXCHANGE-REACTIONS OF MUONIUM ATOMS WITH CHROMIUM(III) COMPLEXES - CONTRASTS WITH POSITRONIUM, Journal of physical chemistry, 98(33), 1994, pp. 8050-8052
Muonium atoms were observed to undergo a fast reaction with several ch
romium(III) complexes in water, with rate constants in the neighborhoo
d of 10(10) M(-1) s(-1). The reaction occurring is attributed to a sim
ple electron-spin-exchange in muonium, catalyzed by the paramagnetic d
(3) metal ions. There is a quantum mechanical statistical factor for t
his process of 62% per collision, which becomes 50% when multicollisio
nal cage effects in solution are included, with the result that the ob
served rates are close to the diffusion-controlled limit for a muonium
reaction in water. The implication is that nearly half of the encount
ers between muonium and a Cr complex result in electron spin exchange.
These muonium rates are faster by a factor of similar to 5 than the c
orresponding reactions of positronium. They also show a different depe
ndence on the nature of the ligands. These differences are attributed
to positronium being larger and more polarizable than muonium, so that
spin-spin coupling with the metal's d electrons is weaker and more se
nsitive to the nephelauxetic effect of the ligands in the case of posi
tronium. The corresponding reactions involving ordinary H atoms are no
t available for comparison but presumably resemble muonium's.