Muonium—the second radioisotope of hydrogen—and its contribution to free radical chemistry

摘要

The involvement of free-radicals in Nature is so widespreadnthat they have entered not only the scientific but the publicnconsciousness. Nonetheless, studying radicals, especially reac-ntive radicals, as they frequently are, is rather difficult. Mostnphysical-organic chemists are familiar with ESR (electron spinnresonance)—now often dubbed EPR (electron paramagneticnresonance)—as a principal means for detecting radicals, butneven this powerful method often lacks the sensitivity required tonaccomplish the task. There are, however, methods available fornthe detection of radicals with enormous sensitivity and speci-nficity which are given the collective acronym µSR (MuSR).nAll the MuSR methods involve a radiolabelling strategy,nsince they rely upon the addition of the light hydrogen atomn“muonium”—a radioactive hydrogen atom with a positivenmuon as its nucleus, having a mass n1n9th that of a protium atom—nto an organic substrate. Muonium may be considered, there-nfore, to be a second radioisotope of hydrogen, after tritium, butnit is much shorter-lived (tritium has a mean lifetime of 17.7nyears, but muonium lives for just 2.2 microseconds!).