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

Abstract

Muon spin rotation and muon level-crossing spectroscopy have been used to determine muon ( A μ) and proton ( A p) hyperfine coupling constants for the muon-substituted tert-butyl radical (CH 3) 2CCH 2Mu over a wide range of temperature in isobutene. A p(CH 3) is almost constant, but A μ(CH 2Mu) falls and A p(CH 2Mu) 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 A μ data cover the temperature range from 297 K down to 43 K, where the solution is frozen. There is a discontinuity in A μ 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 V 2 barrier of 1.8 kJ mol −1, and is consistent with a long CMu bond and a tilt of the CH 2Mu group in the direction that brings the Mu atom closer to the radical centre.