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We have taken the nmr spectra of a series of nitronyl nitroxide radicals with aromatic substituent in order to measure the sign and magnitude of the hyperfine coupling constants and spin densities. These radicals are unusual in that the ortho and para positions of the aromatic rings are found to have negative spin densities. The spin density at the meta position is smaller and positive. A series of molecular orbital calculations were carried out in order to theoretically determine the spin distribution in these radicals. A second-order perturbation calculation was undertaken to analyze the mode of spin propagation from the nitronyl nitroxide ring to the aromatic ring. This calculation yielded a qualitative technique for predicting the sign and relative magnitude of the spin densities in radicals of this type. Hyperfine coupling constants of organic free radicals are generally measured by analysis of electron spin resonance spectra. This technique yields the magnitude (4)

1259liquid just above its melting point, it is likely that many of the nickel atoms are already present in the form of small molecular clusters (2-10 or more atoms) prior to melting. These may form because of surface diffusion of the metal atoms during deposition or bulk diffusion on matrix warm-up. Ozin and co-workers have extensively investigated these factors in the formation of dimers and higher oligomers of transition metals in inert m a t r i~.~~.~~ Vanadium, for example, shows evidence of appreciable dimerization due to surface diffusion in CH4 matrices during deposition at 10-12 K under conditions of quite high dilutionz5 (V:CH4 = 1.5:lOOO). In general, it was found that bulk diffusion becomes important at temperatures around one-third of the matrix melting point.z5 On the basis of this criterion, a large fraction of the nickel atoms must be present in oligomeric form at the melting point of toluene. Since matrix decomposition becomes important only in the liquid phase, and on heating to room temperature, some degree of control of cluster and microcrystallite size might be possible by appropriate choice of solvent to enhance both surface and bulk diffusion rates in the solid during codeposition and subsequent warm-up of the metal/matrix phase.Acknowledgment. The authors wish to thank H. R. Lilienthal, w, E, ~~~1 1 , and R. ~i~~ for their expert assistance with the magnetic, X-ray, and electrical measurements. We are also grateful to Dr. K. J. Klabunde and Dr. P. L. Timms for informative is cuss ions and suggestions. (24) E. P. Knndig, M. Moskovits and G.The EPR spectra of a series of dimeric copper complexes with purine ligands have been investigated to determine dipolar splittings and exchange energies. The copper atoms in these complexes are coordinated to the nitrogen atoms in the purine rings in each case but have different axial ligands. The metal-metal separations and dipolar couplings are similar in this group of molecules, but there are large differences in the exchange energies. The variation in exchange energy with the nature of the axial ligand can be explained by a mechanism involving mixing of the dzz and d, z, z orbitals with overlap of the d9 orbitals of the two copper atoms.

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Angle-selected ENDOR experiments have been performed with polycrystalline samples of bis(2,4-pentanedionato)palladium(II) doped with Cu(II). These experiments allowed the position and Fermi contact energy for a nearby proton to be determined from a randomly oriented sample. The experiments also confirm theoretical predictions of the asymmetric arrangements of ENDOR peaks with respect to the nuclear Zeeman frequency, the g value dependence of ENDOR shifts, and the dependence of ENDOR peak intensities on the electron spin quantum number.