OBSERVATION OF SIMULTANEOUS SYMMETRY-BREAKING OF 2 NONINTERACTING METHYL-GROUPS BY LEVEL-CROSSING NMR

It is reported how rotating-frame level crossing NMR can be used to tr ace the torsional-tunneling ground state manifold of methyl groups emb edded in solid lattices. This tunneling spectroscopy is limited to tun neling frequencies below similar to 800 kHz. The energy levels are ide ntified from the spectrum of Zeeman-torsional level matching resonance s where fast population exchanges take place. To search for such reson ances the proton Zeeman energy splitting is varied by changing, point by point, the magnitude of the effective field in the rotating frame f rom a few G to 50 G. In addition, the tilt of the effective field in t he spin rotating frame is set at any angle, from normal rotating flame (theta B = 90 degrees) to theta = 45 degrees tilted frame. This impro ves the resolution to better than 10 kHz and helps to determine the sy mmetry of the transitions. Observation of a transition between the \A] \A] and \E]\E] states of two noninteracting equivalent methyl groups i n a solid lattice, in which both groups simultaneously undergo a symme try change, is also reported.