RESONANT BROADENING OF NMR LINE-SHAPES OF TUNNELING METHYL-GROUPS AT LEVEL ANTICROSSINGS

At the magnetic field which makes the proton Larmor frequency equal to the rotational tunnelling frequency of hindered methyl groups at low temperatures, terms in the dipole-dipole interactions which are normally non-secular become secular and strongly influence the observed line shape. A study of the low and zero field NMR spectrum is reported for a sample containing groups whose tunnel frequency is 75 kHz. Broadening due to anticrossings occurs at 0.9 and 1.8 mT, in good agreement with calculated spectra. The possible modification of the tunnelling frequency at the resonances is discussed.