ESR Linewidths in Solution. IV. Experimental Studies of Anisotropic and Spin—Rotational Effects in Copper Complexes

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Abstract
The linewidths of copper acetylacetonate in toluene and in chloroform have been measured as a function of temperature (T), viscosity (η), and magnetic field. The contributions arising from motional modulation of the anisotropic dipolar and gtensors have been calculated in terms of independently determined magnetic parameters and a single adjustable parameter, the hydrodynamic radius r (r=3.857 Å in chloroform and 3.357 Å in toluene). The remaining or residual linewidth is independent of nuclear quantum number M and only slightly dependent on the applied magnetic field; it is linear in T/η. A theory of spin relaxation through spin—rotational interaction has been discussed in Article II of this series; the predicted linewidth arising from this spin—rotational interaction accounts very well for the residual linewidth in terms of the single adjustable parameter r already determined. The spin—rotational linewidth is proportional to Σ(gi −2.0023)2, which, in turn, is proportional to the spin—orbit coupling constant squared; the comparison of the residual linewidths of vanadyl and copper acetylacetonate indicate that this dependence is in fact satisfied by the residual linewidths.