The role of substituents on the addition of muonium to aromatics: As seen through muonium-radicals in allyl benzene compared to styrene,and from hammett parameters in aqueous solution

Muonium-radicals resulting from insertion into the benzene ring are found to be much more prevalent in allyl benzene (C₆H₅CH₂CH=CH₂) than in styrene (C₆H₅CH=CH₂). The salient implication of this comparison is that intramolecular rearrangements preceeded the μSR observation for the case of styrene. In turn, this suggests that muonium-containing free radicals, as seen directly by kilogauss transverse field μSR, are not necessarily theprimary radicals. Therefore, the elucidation of mechanism (and identification of the precursor) of Mu-radical formation is further complicated by the fact that the observations may refer to thermodynamically more stable secondary radicals-those resulting from a variety of intra-or inter-molecular relaxations or exchanges. Primary kinetic selectivities of thermalized muonium atom addition reactions can be determined, however, through the substituent effect on the Hammett linear free energy parameter in dilute solution. Results have been obtained for substituted benzenes and benzoic acids. Muonium apparently has a mild nucleophilic character. And, most interestingly, this is opposite to that of its heavy isotope hydrogen.