Tunneling spectroscopy by matching energy levels in the spin-rotating frame

This is the authors' abstract. We don't add key points for this paper.

Tunneling spectra of strongly hindered CH₃ in methylmalonic acid, dimethyl sulfide, propionic acid, and hexane are reported. The Zeeman-tunneling level-matching resonances are detected at ωZ=nωT, n=1/4, 1/3, 1/2, 2/3, 1, and 2 when the level matching is maintained for 10 ms in the 54.7^∘{} tilted proton spin-rotating frame. A ground-state manifold of two noninteracting but equivalent methyl groups accounts for these spectra. All the transitions, which bring about the population equalization whenever a matching resonance occurs, are driven by time-independent dipole-dipole interactions. The resonance peaks at ωZ=2/3ωT and ωZ=2ωT, which are observed in a tilted rotating frame only, indicate that pairs of methyl groups undergo a symmetry conversion simultaneously. The calculated magnetization changes, which are the consequence of population equilibration, reproduce the observed resonance peaks intensities well.