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The Isotropic Hyperfine Interaction 51 possibility of using endor to analyze for different nuclear spin transition selection rules.An analysis of the relevant experimental resultsshowed that they are consistent with both the END mechanism as the dominant "endor-active" process and with exchange processes acting to reduce the endor signals.Acknowledgment. We wish to thank Dr. James S. Hyde for communication of his experimental results prior to publication.

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Electron spin resonance at 3.2 cm has been observed from copper(II) in single crystals of copper(II) bis-acetylacetonate diluted to ½ mole % with palladium(II) bis-acetylacetonate at 77°K and 300°K. The parameters of the spin Hamiltonian have been expressed in terms of the atomic orbital coefficients of molecular orbital wave functions describing the electron hole in the orbital ground state and two excited states. Experimental determination of the spin Hamiltonian then results in expressions for the molecular orbital wave functions. The results indicate that the in-plane sigma and pi bonding are appreciably covalent, whereas the out-of-plane pi bonding with the acetylacetonate ring is ionic. The hyperfine interaction may be expressed by a relation of the type K2g2=A2g∥2cos2θ+B2g⊥2sin2θ+2C2g∥g⊥sinθcosθ, where C2 increases with decreasing temperature. The appearance of the term in sinθ·cosθ is attributed to low symmetry crystal fields due to the crystallographic array, the main symmetry being determined by the molecule itself. A small amount of rhombic character was observed in the g tensor, but was not enough to make an accurate location of the gx and gy directions possible. For the purpose of analysis, the g factor is expressed in terms of the two principal values g∥ and g⊥ which do not vary between 77°K and 300°K to the accuracy of the determination.

B. R. MCGARVEY. Can. J. Chem. 53,2498Chem. 53, (1975. The theory for the spin Hamiltonian parameters of low spin cobalt(II), d7, complexes has been developed to third order in perturbation theory for all reasonable ground states. Two extensions beyond previous treatments have been made: (I) the spin-orbit mixing of excited quartet states into the doublet ground state has been considered and (2) configuration interaction involving mixing of d,z and d,z-,l orbitals in lower symmetry has been included. In the case of axial complexes a more complete calculation has been made to assess the errors incurred in only going to third order in perturbation theory. The theory has been applied to porphyrin and phthalocyanine complexes and is shown to adequately account for the observed data on these complexes. The excited quartet states are much higher in energy than the ground state for the four coordinate porphyrin complexes but become much lower in energy when basic adducts are complexed along the z axis. Further there is mixing of the 4s orbital with the 3d,z orbital for the four coordinate complexes which is completely removed when basic adducts are complexed to the porphyrin system. Application of equations to lower symmetry complexes is considered and it is shown that is is difficult to make an unambiguous assignment of ground state on the basis of g and 59Co hyperfine splitting values alone, although the relative values of gy and g, do seem to provide some clue as to the correct ground state.B. R. MCGARVEY. Can. J. Chem. 53,2498Chem. 53, (1975. La theorie des parametres harniltoniens de spin pour les complexes d7 du cobalt(I1) a spin bas a tte developpk en theorie des perturbations du troisikme ordre pour tous les Ctats fondamentaux. Deux points ont Ct C ttudits au-dela des traitements preckdents: (I) on a considtrt le melange spin-orbite des etats quadruplets excites en doublet de I'ttat fondamental et (2) I'interaction de configuration impliquant le melange des orbitales d,2 et d ,~-~2 dans une symetrie plus basse. Un calcul plus complexe a ete fait dans le cas des complexes axiaux afin d'kaluer les erreurs qui resultent du passage au troisieme ordre de la thtorie des perturbations. Cette thtorie a ett appliquQ aux complexes phorphyrines et phtalocyanines et elle rend bien compte des donntes observtes avec ces complexes. Pour les complexes t~tracoordonn6es de la porphyrine on observe une Bnergie beaucoup plus elevte pour les etats quadruplets excitts que pour I'ttat fondamental. Cette energie diminue quand des ligands basiques sont coordonnts le long de I'axe z. D'autre part, on observe le melange des orbitales 4s et 3d,2 du complexe tttracoordonnt. Ce melange disparait completement quand des ligands basiques sont coordonnes au systeme porphyrine. L'application des equations a des complexes de symttrie plus basse a Ote considtree. I1 n'est pas possible de faire une attribution intquivoque de l'etat fondamental en se servant seulement de g et des valeurs de la scission hyperline du 59Co, bien que les valeurs relati...