Experimental method dependent Zeeman relaxation of nontunnelling methyl groups

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Abstract
The nonexponentiality in the Zeeman spin-lattice relaxation of nontunnelling methyl groups is shown to be closely related to the fact that the relaxation depends on the experimental method. Experimental results on CF3COOAg demonstrate that the half-amplitude times by the saturation- tau -90 degrees method can be, provided the saturating 90 degrees pulses are properly spaced, twice as long as those by the 180 degrees - tau -90 degrees method for the magnetic field perpendicular to the fluorine plane above the temperature of the T1minimum. The exponentiality of the relaxation below the T1minimum temperature, independent of the crystal orientation, implies that the intergroup induced transitions are able to equalize the populations of the degenerate Eaand Eblevels in a time much shorter than any of the time constants describing the Zeeman relaxation. The value 25 kJ mol-1is obtained for the activation energy of the CF3group reorientations. Experiments on polycrystalline acetone show a similar but somewhat smaller dependence on the experimental method.