Observation of simultaneous symmetry breaking of two noninteracting methyl groups by level crossing NMR

Abstract

It is reported how rotating-frame level crossing NMR can be used to trace the torsional-tunneling ground state manifold of methyl groups embedded in solid lattices. This tunneling spectroscopy is limited to tunneling frequencies below ∼800 kHz. The energy levels are identified fromt he spectrum of Zeeman-torsional level matching resonances where fast population exchanges take place. To search for such resonances the proton Zeeman energy splitting is varied by changing, point by point, the magnitude of the effective field in the rotating frame from a few G to 50 G. In addition, the tilt of the effective field in the spin rotating frame is set at any angle, from normal rotating frame ( θ = 90°) to θ = 45° tilted frame. This improves the resolution to better than 10 kHz and helps to determine the symmetry of the transitions. Observation of a transition between the A> A> and E> E> states of two noninteracting equivalent methyl groups in a solid lattice, in which both groups simultaneously undergo a symmetry change, is also reported.