Quantum tunnelling and hopping motion of methyl groups in cadmium acetate dihydrate

Methyl groups in cadmium acetate dihydrate occur in two very different sites, and the rotation of one is much more strongly hindered than the other. The hopping motion of both types has been studied by nuclear spin-lattice relaxation and the tunnel motion of the more weakly hindered type has been very extensively studied by field-cycling tunnel resonance techniques at 4K. The tunnel frequency is 9.9 GHz. Internal resonances between the tunnel frequency and electronic Larmor frequencies of various types of paramagnetic impurities have been detected by observing the field dependence of the nuclear magnetisation recovery after saturation. Examples of impurities interacting with the tunnelling methyl group are Cu 2+ , Cd + and free radicals CH 2 formed by gamma -irradiation. Attempts to measure the tunnel frequency of the more strongly hindered type of methyl groups by similar methods have not been successful. From the nuclear spin-lattice relaxation data, its tunnel frequency is estimated to be about 500 MHz. Calculations indicate that this more hindered type is the one lying close to the planes of the hydrogen bond network which links the parallel Cd-O helices while the weakly hindered type is identified with the groups lying in the channels between these planes.