Hyperfine coupling constants of muonium-substituted cyclohexadienyl radicals in the gas phase: C(6)H(6)Mu, C(6)D(6)Mu, C(6)F(6)Mu

Muon spin rotation (mu SR) and avoided level crossing resonance (ALCR) have been used to determine the hyperfine coupling constants (hfcs) of the muonium-substituted cyclohexadienyl radicals C(6)H(6)Mu, C(6)D(6)Mu and C(6)F(6)Mu in the gas phase, at pressures similar to 1 and 15 atm and temperatures in the range 40-80 degrees C. Equivalent studies of polyatomic free radicals in gases, by electron spin resonance (ESR) spectroscopy, are generally not possible in this pressure range. The present gas phase results support the findings of earlier studies of cyclohexadienyl radicals in the condensed phase, by both mu SR and ESR. Minor but not insignificant (similar to 1%) effects on the hfcs are observed, which can be qualitatively understood for such nonpolar media in terms of their differing polarizabilities. This is the first time that comparisons of this nature have been possible between different phases at the same temperatures. These mu SR/ALCR gas-phase results provide a valuable benchmark for computational studies on radicals, free from possible effects of solvent or matrix environments.