Intramolecular motion in the tert-butyl radical as studied by muon spin rotation and level-crossing spectroscopy

Muon spin rotation and muon level-crossing spectroscopy have been used to determine muon (Aμ) and proton (Ap) hyperfine coupling constants for the muon-substituted tert-butyl radical (CH3)2CCH2Mu over a wide range of temperature in isobutene. Ap(CH3) is almost constant, but Aμ(CH2Mu) falls and Ap(CH2Mu) rises with increasing temperature, consistent with a preferred conformation of the methyl group in which the CMu bond is coplanar with the symmetry axis of the radical orbital. The Aμ data cover the temperature range from 297 K down to 43 K, where the solution is frozen. There is a discontinuity in Aμ at the melting point, as well as a change in temperature dependence. It is suggested that the potential barrier for methyl group rotation is lower in the liquid due to simultaneous inversion at the radical centre, and that the inversion mode is somewhat inhibited in the solid. The best fit of the liquid-phase data indicates a V2 barrier of 1.8 kJ mol−1, and is consistent with a long CMu bond and a tilt of the CH2Mu group in the direction that brings the Mu atom closer to the radical centre.