ELECTRON-SPIN EXCHANGE-REACTIONS OF MUONIUM ATOMS WITH CHROMIUM(III) COMPLEXES - CONTRASTS WITH POSITRONIUM

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.