On the Contribution of the Fermi Contact Term to the Magnetic Field at the Nucleus

1961 ◽  
Vol 32 (3) ◽  
pp. S118-S119 ◽  
Author(s):  
R. E. Watson ◽  
A. J. Freeman
Keyword(s):  
Magnetic Field ◽  
Contact Term ◽  
Fermi Contact ◽  
Fermi Contact Term ◽  
The Magnetic Field

Theory of 13C and 1H paramagnetic shifts in uranocene. Does it indicate f orbital covalency?

1984 ◽  
Vol 62 (7) ◽  
pp. 1349-1355 ◽  
Author(s):  
Bruce R. McGarvey
Keyword(s):  
Nearest Neighbor ◽  
Uranium Atom ◽  
Paramagnetic Shift ◽  
Contact Term ◽  
Covalent Bonds ◽  
Dipolar Contribution ◽  
Contact Shift ◽  
Polarization Mechanism ◽  
Fermi Contact ◽  
Fermi Contact Term

The paramagnetic shift resulting from f orbital covalency for lanthanide fluorides has been successfully treated theoretically in earlier studies. This theoretical method has been modified to calculate the paramagnetic shift for 13C and 1H in uranocenes. It is shown that the very large anisotropy in the magnetic moment makes the treatments in the literature incorrect. The so-called "contact shift" in 13C is in reality three terms: (1) a dipolar contribution from spin in the carbon pz orbital, (2) a dipolar contribution from spin in the nearest neighbor carbon pz orbitals, and (3) an induced Fermi contact term. Contribution (1) is the largest. For 1H there are contributions from the direct dipolar interaction of spin in the nearest carbon pz orbital and the induced Fermi contact term. Both terms are nearly equal in magnitude. The theory, however, predicts a much smaller magnitude and the incorrect sign for the shift as measured experimentally. The measured shift must, therefore, result from a polarization mechanism in which exchange interactions between the f electrons and electrons in filled orbitals cause spin delocalization into the ligand σ orbitals. It is argued that this polarization mechanism operates primarily through the overlap of the outer 6s and 6p orbitals of the uranium atom with the σ orbitals of the ligand making the observed nmr shifts insensitive to the nature of the bonding between the ligand and the uranium atom. We conclude, therefore, that the question of how much f orbitals are involved in covalent bonds between uranium and the ligands in uranocene cannot be answered by measuring the paramagnetic shift in 13C or 1H.

EPR Studies of Cu2+ in dl-Aspartic Acid Single Crystals

1999 ◽  
Vol 54 (3-4) ◽  
pp. 256-260 ◽  
Author(s):  
B. Karabulut ◽  
R. Tapramaz ◽  
A. Bulut
Keyword(s):  
Single Crystal ◽  
Single Crystals ◽  
Aspartic Acid ◽  
Ground State ◽  
Contact Term ◽  
Mixing Coefficients ◽  
Fermi Contact ◽  
Fermi Contact Term

EPR studies of Cu2+ doped dl-Aspartic Acid [NH2CH(CH2COOH)COO] powder and single crystal have been carried out at 113 and 300 K. The principal hyperfine and g values, covalancy parameter, mixing coefficients and Fermi-contact term of the complex were obtained, and the ground-state wavefuntion of the Cu2+ ion in the lattice has been constructed.

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