Brodovitch, J.-C.

The muoniated methyl radical has been detected in a sample of liquid ketene irradiated with positive muons. The muon and proton hyperfine coupling constants were measured at 184 K by transverse field muon spin rotation and avoided muon level-crossing resonance, respectively. After correction for the differing magnetic moments, the muon hyperfine constant is only 3% larger than that of the proton. This small isotope effect can be attributed to zero-point motion in the anharmonic C-H (C-Mu) stretch.

We report on recent results obtained for longitudinal field (T 1) spin relaxation of the muonium-substituted (“muonated”) free radicals MuCO, MuC2F4, MuC2H3F, and MuC4H8 (t-butyl), comparing with results reported earlier for MuC2H4 (and MuC2D4). Some comparison with transverse field (T 2) data is also given. These data are fit to a phenomenological model based on NMR theory of spin relaxation in gases. The parameters of these fits are presented and discussed.

The radio-frequency muon spin resonance technique (RF-μSR) is described, with examples drawn from muon studies of fullerences. Two distinct species can be detected by RF-μSR when solid C60 is irradiated with positive muons. Endohedral muonium (Mu@C60) is characterized by a muon hyperfine constant (A μ) close to the vacuum value. A remarkable feature of the RF-μSR spectrum is the double quantum transition, which appears when the allowed transitions are saturated. The exohedral muonium adduct (C60Mu) is also detected, and has a much smaller value ofA μ typical of a carbon-centred organic radical. It has been studied by RF-μSR in dilute solution, which is not possible for transverse field muon spin rotation (TF-μSR). There is a significant difference inA μ of C60Mu in the solid and in solution, a result of great import to the analysis of avoided-level crossing experiments on13C60Mu.

Muon hyperfine constants A_μ have been measured by transverse field μSR for (CH3)3Si\mbox\.CHMu in hexane from 167 K to 332 K. In addition, avoided level‐crossing resonance was used to determine \alpha‐proton coupling constants Ap over a similar range of temperatures. The two hyperfine constants can be described by a common temperature dependence, d|Ai|/ dT=1.4\times 10-3 MHz\,K-1, where Ai represents Ap or the reduced muon constant A^\prime_μ=0.3141A_μ. There is a small isotope effect (A^\prime_μ is 2.2 % larger than Ap) consistent with zero‐point motion in the anharmonic C–H bond stretch. The common temperature dependence is tentatively attributed to a coupled deviation of the C–H and C–Mu bonds out of the nodal plane of the p orbital containing the unpaired electron.

Muon irradiation of pure liquid 3‐chloropropene, CH2=CH-CH2Cl, yields a primary radical, \dot\mboxCH2-CHMu-CH2Cl, and a secondary radical, MuCH2-\rm\dot\mboxCH-CH2Cl. 2‐methyl‐3‐chloropropene yields only the tertiary radical, MuCH2-\rm\dot\mboxC(CH3)-CH2Cl. These three chloroalkyl radicals have been characterized by μSR and μLCR, and the hyperfine coupling constants (hfcs) have been determined over a range of temperatures, either in the pure liquid precursor or in concentrated solution. The temperature variation of the hfcs has been analyzed to obtain estimates of the barrier to internal rotation about the C_\alpha-C_\beta axis for various alkyl groups, and also their minimum energy conformations, i.e. their orientations with respect to the axis of the 2p_z orbital of the unpaired electron. The tertiary radical is particularly interesting because all three methyl‐like groups, -CH3,-CH2Cl and -CH2Mu, are represented. The results can be compared to electron spin resonance data for analogous radicals, to provide information on the effects of Mu substitution for H.