Hydrogen and muonium atom adducts of trimethylsilyl derivatives of ethyne

Abstract

Exposure of dilute solutions of Me₃SiCCH in CD₃OD to ⁶⁰Co γ-rays at 77 K gave mainly H(D)CĊSiMe₃ radicals together with traces of H₂CCSiMe₃. The relative concentration of the latter increased markedly on annealing, presumably via proton transfer. The results suggest that electron addition is followed by protonation and hence that the anion of HCCSiMe₃ is strongly basic. The β-proton hyperfine coupling of 59 G for the monodeuterio derivative is equal to that for the two β-protons in H₂CCSiMe₃, and also equals the ²H coupling after correction for the difference in proton/deuteron magnetogyric ratios. Hence the radical must be linear, the SOMO being, nominally, the in-plane 2p orbital on the carbon atom bound to silicon. The β-proton coupling for the radical Me₃Si(H)CĊSiMe₃ formed similarly from Me₃SiCCSiMe₃ is much greater (76 G), suggesting that this radical is bent, with a trans configuration, favouring overlap with the C—H bonding orbital. Exposure of the pure liquids to a beam of positive muons gave the corresponding muonated radicals, H(µ)CĊSiMe₃ and Me₃Si(µ)CĊSiMe₃. The muon–electron hyperfine couplings, corrected for the difference in the magnetic moments, were both greater than the corresponding proton couplings by a factor of 1.14; reasons for this enhancement are discussed.