Structure of muoniated trimethylsilylvinyl radicals

Muoniated vinyl radicals can be produced by muonium (Mu) addition to triple bonds. Rhodes et al. observed a muoniated radical formed by Mu addition to trimethylsilylacetylene but were unable to determine the structure (J. Chem. Soc., Chem. Commun. 1987, 447). We have performed transverse field muon spin rotation measurements on trimethylsilylacetylene at 298 K and two different applied magnetic fields and observed that two types of muoniated radical were formed: a major product with muon hyperfine coupling constant (A(mu)) of 587.3(1) MHz and a minor product with A(mu) of 570.9(5) MHz. DFT calculations show that Mu addition is preferred to the terminal carbon of the triple bond and that the two resulting radicals are isomers of the Me3SiC=CH(Mu) radical. Calculated muon hyperfine coupling constants were used to assign the observed radicals, which have Mu cis and trans to the trimethylsilyl substituent, respectively.