Abstract
Muonium-substituted free radicals are observed by muon spin rotation when positive muons are stopped in liquid methyl, and fluorine-substituted benzenes. From muon precession frequencies in high external magnetic fields the isotropic muon-electron hyperfine coupling, constants A gm are determined. They are typical of cyclohexadienyl-type radicals. The individual assignments are based on deuteration and on substituent effects. Comparison of A gm with A p of the hydrogen analogues reveals isotope effects A μμ p/ A pμ μ of 1.15–1.21. Analysis of signal amplitudes yields substituent effects on the relative rates of Mu addition to inequivalent positions in the arenes. Both CH 3 and F are ortho directing. Addition at CH sites is favored over ipso addition by a factor of ≈ 3.
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